"So it's like this new nervousness: How do you change things up?  Because you don't want to offer the same experience, just skinned with next-gen graphics.  But then you don't want to remove something that everybody loved and [make] the new one suck ... It's just been this odd thing I haven't felt in many years." 

"I remember saying to myself [while making Mortal Kombat 2] 'We're never gonna sell six million units like we did the first one,'" Boon tells Polygon. " I wouldn't have guessed that, but then I find myself in the same situation we were in 20 years ago, which was 'Oh shit, we're following up this crazy success.' 

The next Mortal Kombat has to accomplish something that makes series co-creator Ed Boon a bit nervous.  It has to be better than the best-selling Mortal Kombat game of all time, the 2011 franchise reboot, Mortal Kombat. 

Three times the fun Mortal Kombat X plays much like its predecessor.  Players still fight in bloody one-on-one battles, there's still a block button and features like the bone crunching X-ray moves, breakers and enhanced special moves, all powered by a meter, return. " A lot worked in MK9 and we certainly don't want to throw all that just out to throw it out," Boon says. " We have a great foundation of what worked with the game and we're layering something new on top."  With Mortal Kombat X, coming to PlayStation 4 and Xbox One next year, Boon and the rest of NetherRealm Studios are changing things up with a major alteration to how characters play.  In Mortal Kombat X, each fighter will come with three variations.  A classic character like Scorpion, for example, will still have his trademark spear attack.  But one version of Scorpion will be able to summon a demon companion that can dive kick an opponent or grab them from below.  Another version of Scorpion can fight with swords, while a third can self-immolate, burning his opponent when they're nearby. 

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Scorpion's longtime nemesis Sub-Zero will always have his slide attack and freezing projectile.  But the "Cryomancer" variation will let Sub-Zero create ice swords from thin air, while the "Grandmaster" variation will let him form ice clones that can serve as a shield or be thrown as a projectile.  Boon and team tried something similar with 2002's Mortal Kombat: Deadly Alliance.  In that game, players could choose from a variety of weapons and martial arts styles, but Mortal Kombat X's focus is on each character's suite of special moves.  Some of Mortal Kombat X's new fighters feature the most interesting variations.  D'Vorah, a yellow insect woman, can fire larvae as projectiles in one of her variations, while another variation adds poison to her stabbing attacks.  A new character Ferra/Torr — a combination young woman, giant brute fighter — can be played with variations where Ferra (the girl) can attack her partner (Torr) to enrage him or in which she's not part of the fight at all.  Torr, the giant, will play by himself while Ferra stands on the sidelines in that one. " We had to bring something to the table that fighting games haven't done before," Boon says, "and answer the question 'Why do I give a shit about this Mortal Kombat?'  That's where the character variations came in."  Each fighter's variations will also be represented visually in subtle ways.  Cassie, for example, wears a pair of sunglasses in her "Hollywood" variation — the fighting style that features her "Nut Kracker" groin punch.  In her "Spec Ops" variation, she wears an earpiece that lets her call in airstrikes.  The combination of character and movelist variations, Boon hopes, will add greater depth to Mortal Kombat X's fighting system.  Instead of having to worry how Scorpion and Cassie match up against each other, players will now have to consider how well an Inferno-style Scorpion and a Brawler-style Cassie match up, or whether a Hellfire-style Scorpion is the better option.  That leads to a major balancing challenge for NetherRealm, something Boon is clearly aware of. " We're going to have tons of [competitive players] coming in and just hammering on the game," he says. " We have so many more variables in the mix, we need to get those guys in as much as possible.  It's great to have this big canvas, but we want have it balanced." 

Other changes Further changing the Mortal Kombat formula, the fighting arenas in Mortal Kombat X will be interactive, similar to those in NetherRealm's last game, Injustice: Gods Among Us.  Combatants will be able to grab objects, like tree branches or barrels, from the backgrounds and toss them at opponents.  A particularly cruel example is in the Outland Marketplace stage: fighters can grab a little old lady from the background and use her as a projectile.  Combatants will also be able to jump off certain objects — a tree stump, a tent — to get themselves out of a corner quickly or rush down an opponent.  In the game's frozen forest stage, these jumps can actually be chained together, allowing players to quickly leap across the entire stage.  Those moments of interacting with the environment are limited by a small stamina meter below each fighter's health bar, which has been designed to limit overuse of those moves.  Another change to stage interactivity, Boon says, is that thrown stage objects can be blocked, unlike in Injustice.  Adding all-new characters to the mix, Boon says, was another important step forward for the franchise.  After Mortal Kombat, which included a roster pulled almost entirely from the first three Mortal Kombat games, the team wanted to design new ones. 

"We're taking an aggressive approach with new characters," Boon says. " We're going to introduce a number of new ones.  It's going to be a really very different experience. " It's hard because there's 60-something characters in the MK universe to choose from.  But we made a choice to have a good representation of new characters."  At E3, NetherRealm and Warner Bros. are showing off six characters.  Two are veterans — Sub-Zero and Scorpion — but the rest are new.  In addition to Ferra/Torr and D'Vorah (Boon's current favorites), the Mortal Kombat X demo features Cassie Cage — the daughter of Johnny Cage and Sonya Blade — and Kotal Khan — a demigod with an Aztec style.  Those new characters represent a shift for the franchise.  While Boon and company aren't spilling many details about the story, it's clear that the game takes place some time after the events of the last Mortal Kombat.  As the franchise makes the leap to the next-generation of consoles, so too does it transition to a new generation of fighters.  The visual design of those fighters will steer more toward "simplicity and elegance," Boon says. " We didn't want to get too complex [with their design]," he adds. " We wanted big variance in materials [and] really wanted to get leather, cloth, worn out materials in there."