List of unreleased mechanics

There are some mechanics that have been designed by R&D but have been never released for several reasons as rules concerns. Some of them have been publicly explained over the years.
==Unreleased mechanics==
*"Accelerator" for Tempest. Spell that gave an opponent a poison counter, but only if they already had one. The mechanic, as designed for Tempest, was a one-off effect and not an upkeep ability on permanents.
*"Artifactfall" for Kaladesh. Effects that trigger whenever an artifact you control enters the battlefield.
*Advocate for Commander Legends. A triggered mechanic (like landfall or constellation) that triggers whenever your commander enters the battlefield or attacks. 
*Anchor For Time Spiral. Big creatures for cheap that required a mana payment for upkeep.
*Ballistic Skill For Warhammer 40,000 Commander Decks a new stat, besides power and toughness.
*Bribe for Conspiracy: Take the Crown and Commander Legends. The spell gives you an effect. Then, each of your opponents may choose to draw a card. For each one that did, you get the effect again.
*Call for Ixalan. A Dinosaur with call was usually a bigger Dinosaur, but one with a very cheap mana cost. You couldn't attack or block until you paid a one-time activation cost. 
*Camouflage (As long as this is untapped, it can't be the target of spells or abilities your opponents' control) for Ikoria: Lair of Behemoths. This is similar to <c>Paradise Druid</c>. 
*Cartel For Streets of New Capenna. A creature version of metalcraft (spells that got an upgrade if you controlled three or more creatures).
*Celestial (You may cast this card from Nyx. If it would enter the battlefield any other way, it goes to Nyx instead. If the card would leave the battlefield, it goes to Nyx instead. Your devotion to white reduces its cost by that much N) for Theros.
*Clan for Kaldheim. A modified version of Leader adapted for use as a tribal mechanic. When you played a clan card, you chose a creature type, and the card affected that creature type. Then when you played a second clan card, you could rechoose your creature type, but both effects would affect the one chosen tribe. 
*Conjure [creature type] – [cost] for Strixhaven: School of Mages (If you cast this spell for [cost], when it enters the battlefield create an [appropriately sized, colored, and creature-typed] creature token.)
*Cooperation (Whenever this creature deals combat damage to a player, put a +1/+1 counter on another target creature.) for Throne of Eldraine. A variant of the Slith mechanic.
*Dead or Alive for Outlaws of Thunder Junction. Kill something now or put an enchantment on it that does something incremental.
*"Deathfall" for Jund in Shards of Alara. Effects that trigger every time a creature dies.
*Debt for Orzhov in Ravnica Allegiance. Spells give debt counters to opponents. At the beginning of their end step, they can get rid of any number of debt counters by paying per debt counter. Then, as long as they have any debt counters remaining, they lose 1 life (not per debt counter just 1 life total regardless of how many debt counters they have). Player counters that caused life loss were worked into radiation and rad counters. Later reflavored (but still unreleased) as Injure for Duskmourn: House of Horror. 
*Defeatable for Bloomburrow. Damage dealt to this creature is done in the form of -1/-1 counters.
*Discipline N (Whenever this card becomes blocked, put N +1/+1 counters on this card.) for Throne of Eldraine.
*Dig [cost] for The Lost Caverns of Ixalan. [Cost], Discard this card: Surveil 1, then draw a card.
*Disguise for Dimir in Guilds of Ravnica. A ninjutsu variant where the creature with disguise is swapping with the attacking creature. 
*Double-tap Keyword action for Time Spiral. Cards that could tap, then tap again. Led to exert.
*Endless for Future Sight. A keyword that made the creature return to your hand whenever it went to the graveyard from the battlefield.
*Enhanced for Kaldheim. Enhanced meant that a creature was either enchanted or equipped. The ability went on creatures and gave them an extra ability if they were enhanced. Later reworked into modified.
*Enlist for Ixalan. Pay extra mana to create a creature token.
*"Enter the Upper City" For Battle for Baldur's Gate. Loosely inspired by King of Tokyo-esque battles for dominance and the three-tiered structure of Baldur's Gate itself. Through combat and keyword actions, players would move up or down a map of the city, accruing rewards for elevating their status to the top and maintaining it there at the expense of other players.
*"Epic Feats" for Bloomburrow. Enchantments with a small enters-the-battlefield (ETB) ability up front and a large effect you hope to achieve in the late game.
*Familiar – [Effect] for Strixhaven: School of Mages (. . . if you control a creature token).
*Fellowship for Bloomburrow. [creature type] joins your Fellowship. Customized batching. 
*Fickle for Tempest. Pay Fickle cost during upkeep or spell returns to owner's hand.
*Finale For Rakdos in Ravnica Allegiance. The creature gains +N/+0 and some abilities, in exchange for being sacrificed at the end of turn.
*Floating By Mark Gottlieb, for Commander (2011). Appeared on creatures and allowed you to summon creatures under another player's control and they would be unable to attack you. Inspired the cycle of Vow auras in Commander 2011.Untold Stories of the Original Commander Decks w/ Melissa DeTora! | Commander Chronicles: 2011
*Foul play for Murders at Karlov Manor. "Whenever a creature card or creature token is put into your graveyard from anywhere, this creature gets -1/-1 until end of turn." 
*Forbidden By Mark Rosewater, for Avacyn Restored. The card starts the game in exile.
*Glorify For Amonkhet. If this creature dies when attacking, it grants a bonus to another creature.
*Gunk By Richard Garfield. Blank cards that didn't do anything but could be transferred to the opponent's deck.Mark Rosewater, Drive to Work #214 "2008" It later appeared on a test card in the Mystery Booster set. 
*Heft the value of combined power and toughness.
*Hedronize For Battle for Zendikar. Roll a d8 and trigger a different effect depending on the value rolled.
*Hieroglyphics By Mark Rosewater, for Amonkhet. Pay to exile the card from the graveyard and draw a card.
*Illuminated for The Lost Caverns of Ixalan. When [this creature] dies, it deals X damage to each opponent, where X is the number of colors among permanents you control.
*Implements for Strixhaven: School of Mages.. Artifacts with two different tap abilities (one in each color of the school) and a third ability which was the combination of the two abilities. 
*Injure / Injury for Duskmourn: House of Horror. (Create an Injury enchantment token with enchant player and "At the end of the injured player's turn, they may pay {o2} any number of times to remove that many injuries. Then if they're still injured, that player loses 1 life."). Same mechanic as Debt.
*Inventions for Kaladesh. Getting things from outside your deck. It was like lesson/learn except that they were all artifacts rather than instants and sorceries.
*Jewel For Guilds of Ravnica. It went on any instant or sorcery. After the card was resolved, it got exiled. Then, whenever you cast another instant or sorcery, you could play the first card for free from exile. The idea behind it was that we built smaller effects that combined to do cool things. 
*Joust (When this creature is blocked, it gets +0/+N until end of turn. If not blocked, it gets +N/+0 instead.) for Throne of Eldraine.
*"Land drop" for Zendikar. : action (You may use land drop only if you haven’t played a land this turn. If you use land drop, you may not play a land for the remainder of the turn.)
*Leader for "Salad", an unmade set intended to follow on from Dominaria. Whenever you played a leader card, you chose a leader from among your creatures, and that card granted an ability to your leader. If you played a second leader card, you had the option of changing who your leader was, but both cards would grant abilities to a single leader. Later evolved into The Ring Tempts You.
*Master Plan for Outlaws of Thunder Junction. When this creature enters the battlefield, you may exile an instant or sorcery card face down with it. When CARDNAME deals combat damage, you may cast that card for 2 less.
*Mechanic I For Kaladesh. The Invention unreleased subtype (see below).
*Megaparty For Battle for Baldur's Gate. Batching all the Class creature types.
*Mindhack For Streets of New Capenna. It went on creatures, and it allowed you to have the creature enter tapped and get a looting effect (draw a card and discard a card). 
*Mining for The Lost Caverns of Ixalan. Exiling colored cards for Artifact counters (pearls, sapphires, jets, rubies, and emeralds). Similar to Energy but for one color.
*Mummify For Amonkhet. As embalm or eternalize, but makes the creature a 2/2 Zombie with wither.
*"Negative cantrip" For Tempest. The inverse of a slowtrip, it forced target player to discard a card at the start of their next upkeep. Playtesting showed the mechanic was extremely unfun.
*Nostalgia By Mark Rosewater, for Time Spiral. It allowed you to put Vintage-legal cards into your deck that you might not normally be able to play.
*Notoriety, becoming notorious for Outlaws of Thunder Junction. Notorious N (At end of turn, if this creature is nonlegendary and has dealt N or more combat damage, put two +1/+1 counters on it and it becomes legendary.).
*Oiled for Phyrexia: All Will Be One. E.g. Oiled 4 (CARDNAME enters with four oil counters and gets an oil counter when empty if you proliferate.)
*Paincast For Rakdos in Return to Ravnica. Spells 1 mana cheaper for each point of damage you had dealt to an opponent this turn.
*Plunder For the Vampires and Pirates of Ixalan. Based on bloodthirst. When you cast these cards, if you had dealt combat damage to an opponent, the card received some kind of bonus. Remained unkeyworded, but revisited as the Vampires' theme in Midnight Hunt.
*Posse for Outlaws of Thunder Junction. Enchant multiple creatures.
*Precedence For Azorius in Ravnica Allegiance. When this enters the battlefield, copy the ETB ability of any other creature you control.
*Reckless For Gruul in Gatecrash. An ability that sacrificed the creature at the end of the turn when triggered.
*Relentless For Time Spiral and inspired by <c>Relentless Assault</c>. Allowed creatures to attack twice each turn.
*Relentless for Phyrexia: All Will Be One. If you cast this spell from your hand, exile it as it resolves. You may cast it from exile for its relentless cost if you've played a land that turn. (not to be confused with Relentless.
*Reveal For Tempest. Allowed the card to be revealed to all players when drawn for an additional effect. It may have serves as the inspiration for Miracle.
*Reverse Engineer For Kaladesh but pushed for Aether Revolt. It allowed you to copy an artifact and then that copy got sacrificed at end of turn, in the same way <c>Heat Shimmer</c> from Lorwyn does for creatures.
*Robbery for Outlaws of Thunder Junction. Kicker where you steal opponent's cards.
*Roar By Mark Gottlieb, for the Dinosaurs of Ixalan. "You may pay [cost] and exile this card from your hand. If you do, you can still cast it this game. When CARDNAME roars .... [effect]". Led to Adventure.
*Rumble For Streets of New Capenna. A rider on spells that allows you to pay 2 life to get a 1/1 creature token.
*Scrap for The Brothers' War. Scrap N (N, Exile this from your graveyard as a sorcery: Target artifact gains all its other abilities permanently.)
*Sense [card type] N for Bloomburrow]]. Look at the top 5 cards of your Library. You may put up to N [card type] cards into your Hand. Put the rest of them on the bottom of your Library in random order.
*Shifting loyalties for Outlaws of Thunder Junction. Changing card type, put on other player's creatures.
*Shoot-out for Outlaws of Thunder Junction. You and target opponent simultaneously reveal a card from your hand or the top of your library. If you revealed the higher mana value card, you win the shoot-out.
*Showoff By Mark Rosewater, for Lorwyn. It allowed you to choose to reveal a card. If the card was revealed (from your hand or your library), then it could be played for its showoff cost, which was often cheaper than its normal mana cost.
*Sift for Dominaria United. Sift was a cycling variant that allowed you extra utility out of the graveyard. Sift COST (COST, Exile this card from your hand or graveyard: Scry 2. If it was exiled from your hand, draw a card.).
*Skilled / Feat For Battle for Baldur's Gate. A Skilled Commander started the game with a feat from your deck. Feats were kind of a monstrous emblem that initially showed up on all the mono-color legendary creatures. You could pay their cost to permanently turn them on and grant your commander that ability for the rest of the game, even if it left the battlefield and returned. In addition, if your commander had the skilled ability, you could choose the feat of another card and put it into your command zone. Then once your commander was on the battlefield, you could spend the cost to permanently turn it on. Your commander having a feat added that color to your commander's color identity. Skilled creatures were allowed to use the feat that was on them.
*"Sliding" designed by Richard Garfield for Tempest. When you draw card with Sliding, reveal it to all players and put it on the bottom of your library to draw another card at the start of the next turn's upkeep. A prototype of Cycling, this mechanic combined Tempest's slowtrips with the unreleased Reveal mechanic.
*Smuggling for Streets of New Capenna. A way to help with color fixing.
*Spellback for the Izzet League in Guilds of Ravnica. Somehow allowed players to "mix and match various instants and sorceries".
*Spring Trap A keyworded version of the Trap mechanic for Zendikar. This version of the mechanic allowed players to exile a card with the mechanic from their hand face down with an arm counter on it, then play that card for a reduced cost as long as it was armed.
*Stopwatch mechanic forces you to do certain actions in a set amount of time. Considered for Unstable, cut down to two cards.
*Spell-pair For Battle for Baldur's Gate. A commander mechanic for enabling players to add color to their mono-color commander by starting the game with an instant or sorcery in the command zone that they could cast once per game, possibly under a specific condition.
*Study – [cost] for Strixhaven: School of Mages. A mechanic where you can spend a resource (tapping at common, mana at uncommon) to add an ability to the card. Each card with study has two options of abilities to add. The second time you activate it, you can add the ability you didn't choose the first time.
*Stygian for Theros Beyond Death. When you play a card with the stygian mechanic, it brings onto the battlefield a special card from outside the game that represents the river of the dead. Creatures on the living side could only block creatures also on the living side while creatures on the dead side could only block creatures on the dead side.
*Titan for Zendikar Rising. A kicker variant where the mana cost was always generic and always seven or more mana. It tended to mega-juice the spell. The flavor was that you were tapping into some residual Eldrazi energy that was seeped deep into the land. The mechanic was templated in a similar manner to Awaken so that players wouldn't need to mentally calculate the total cost of the spell.
*Torment Each opponent loses 3 life unless that player sacrifices a non-land permanent or discards a card. Reduced to an unkeyworded vertical cycle in Hour of Devastation.
*Turmoil For Gruul, then Rakdos in Ravnica Allegiance. An effect that triggered at the end of your turn if an opponent had lost life during that turn.
*Unique By Mark Rosewater, such a mechanic is intended to narrow the Legendary rule effect which would be applied only to legendary permanents having the Unique ability.
*Unnamed By Mark Rosewater, for Tempest. It allowed you to choose to start with the card in your opening hand. If you chose to do so, you had to begin with one card fewer.
*Unnamed For Amonkhet. It made use of -1/-1 counter to represent the ruthlessness of Bolas.
*Unnamed For Champions of Kamigawa. A mechanic representing martial art moves.
*Unite for Dominaria United. A monstrosity variant flavored as two creatures teaming up. When you paid the mana to add +1/+1 counters, it also added color and a creature type.
*Valiance (Whenever this creature deals combat damage to a player, put a +1/+1 counter on it.) for Throne of Eldraine. This is the Slith mechanic.
==Unreleased colors==
*Purple For Planar Chaos.
==Unreleased lands==
*Barry's Land By Mark Rosewater, for Invasion. Later by Bill Rose, for Conflux. Printed as a test card in the Mystery Booster set.
*Cave/City See Purple.
==Unreleased card types==
*Structures designed by Richard Garfield for Ravnica: City of Guilds. The idea was that they represented buildings. Structures functioned a lot like enchantments or global artifacts, in that they had a static effect, but they had one difference — a toughness that the opponent could attack and whittle down. Elements of them went into making planeswalkers and battles.
==Unreleased subtypes ==
*Quest (other mechanic than the released Quest enchantment) For Zendikar and Throne of Eldraine. Enchantments that gave you three tasks. You marked them with counters as you finished each one. Once all of them were completed, you could sacrifice the card for a big effect. Some got you the legendary artifacts for each court. Featured on the Mystery Booster 2 test card .
*Holiday for Bloomburrow. Subtype for certain Instants and Sorceries that made them fetchable.
*Invention for Kaladesh. Cards that let you fetch artifact cards from outside the game. The only unexplained mechanic of the five that the Kaladesh design team ended up making and liking. It got discarded for excess of mechanics.
*Scroll for Strixhaven: School of Mages. The idea of scrolls was that they were artifact tokens that could hold an instant or sorcery in them. You then sacrificed them to cast the spell it was holding. Some cards let you make scrolls out of available instants or sorceries. Other cards made scrolls with spells (usually famous ones from Magic's past) already inside them.
*Several creature types designed for Unglued 2: The Obligatory Sequel
**Teacher
**Torturer
**Vegetable
==Unreleased supertypes==
*Über-classes By Mark Rosewater, for Morningtide. Cards that affected "fighters" affected Soldiers and Warriors (and possibly Knights). "Mages" were Shamans and Wizards (and possibly Druids). "Scoundrel" meant Assassins and Rogues. Similar to the later batching.
==Unreleased zones==
*Midway for Unfinity. A section on the battlefield that any player can use.
==Unreleased counters==
*Volatile counters By Mark Gottlieb, for Aether Revolt. When a card with a volatile counter dies, it deals 2 damage to target opponent.
*Revenge! counters for Outlaws of Thunder Junction. Temporary thieving, theft counters that last for some number of turns
*Theft counters for Outlaws of Thunder Junction. Revenge Cost (Cost, exile this from your graveyard: Do an effect.) 
==Unreleased tokens==
*Scroll tokens for Strixhaven: School of Mages. Artifact tokens that had a spell “inside”. You could sacrifice them to cast the spell.
*Artifact tokens (pearls, sapphires, jets, rubies, and emeralds) for The Lost Caverns of Ixalan. Similar to Treasure except that they only sacrificed for a single color.
==Used mechanics==
These formerly unreleased mechanics were eventually used in new sets.
*Assemble Printed on a single card in Future Sight, but without any associated mechanical definition. Released in Unstable.
*"Creaturefall" for Selesnya in Guilds of Ravnica. Effects that trigger whenever a creature you control enters the battlefield. Released as Alliance in Streets of New Capenna. 
*Last strike By Mark Rosewater, for Future Sight. Released in Unstable.
*Link By Ken Nagle, for New Phyrexia. Supposedly the same [Redacted] mechanic that Aaron Forsythe hinted. Released in Eldritch Moon as meld.
*Mechanic E By Mark Rosewater. Mechanic E, "energy", was originally designed for Mirrodin and finally printed in Kaladesh as energy counters.
*Planeshift (Pay the casting cost the upkeep after creature is summoned or bury it.) For Tempest. The mechanic would see print as Echo in Urza's Saga.
*Poisonous For Tempest. The original design of Poisonous was written and appears to function more like Toxic than the version released in Future Sight.
*Pwnage By Ken Nagle, for New Phyrexia. It allowed you to shuffle cards from your opponent's library into your own. It was killed by the rules, but released as <c>Praetor's Grasp</c>.
*Plot For Amonkhet. A suspend variant representing you planning ahead. Released in Outlaws of Thunder Junction.
*Q-mechanic By Mark Gottlieb, for Shadowmoor. It became the untap mechanic ().
*Resistance (This creature isn't a legal target of a spell or ability an opponent controls unless they pay as that spell or ability resolves) for Ikoria: Lair of Behemoths. Led to Ward.
*Skirmish for War of the Spark. "Create a Skirmish if one hasn't been created yet." The Skirmish token has a tiny "game board" on it showing a field of combat. When you perform one of the stated conditions, you advance on the field toward the opponent's side. If you have advanced enough, you win the skirmish. There is a payout for winning, a generated effect. Released in Dungeons & Dragons: Adventures in the Forgotten Realms as Venture into the dungeon.
*"Sneaky" (This creature can't be blocked by creatures with power 3 or greater) for Ikoria: Lair of Behemoths. The mechanic would latter be used on the Mystery Booster 2 test card , where it was keyworded as Nimble.
*Triple strike By Mark Rosewater, for Future Sight. Released in Unstable.
==References==