Space Engineers - I want to mod it. It's been released for 7 years now - Keen Software has been keeping it intact and adding features (VRage). I think of it as Minecraft but with a near-true physics engine.

There is terrain deformation according to the force exerted by the object crashing into it. Ships colliding will either destroy or crush blocks according to that force. A "grid" is considered a powered assimilation of blocks, which could function as static "station" or moving vehicle. Thrust is measured in Newtons, block locations are measured in Cartesian coordinates. Everything has precise mass (kilogram) and this effects collision damage. Someone made a mod where there are aerodynamics (atmospheric force on fast traveling bodies, though this is computationally expensive). There is already a large workshop of mods that other players have made.

I say "near-true" physics because gravity of planets only stretches out a few kilometers above the planet. After that there is "zero" gravity and you can turn engines off and stay in that spot forever. Ship speeds are capped at 100 m/s. These limitations are required for now because computers cannot keep up with that many calculations especially if grids are interacting and causing terrain deformation or casting functions often.

There are sim speed drops during large scale events, and that is a challenge that draws me in. Keen uses rendering tricks for view distance draw and object visibility according to user settings (reflecting the power of their PC), eventually I'd like to standardize things so that players could interact planet-side without trees disappearing and nullifying camouflage attempts.

The vanilla game requires you to find and extract ore (iron, magnesium, nickel, platinum, cobalt, silver, gold, uranium, more could be added if you wish). These elements can produce things that somewhat reflect their utility IRL, then they are refined using energy (solar, wind, battery, nuclear, hydrogen). Then produced into components that you weld into the block for its completion.

When you join an official server right now, you are confined to a max speed, as this is a safety measure to prevent the game from crashing. A server sets up conditions for players to build in their secret remote spot away from everyone else, though sometimes they will group into factions. There is seldom any interaction and NPC ships are randomly spawned in near the player, flying in a straight line - they have no deeper function and they are meant to be your salvage.

My big picture is that there should be no speed limit, but players locations give off visual signatures if they want to go fast all the time. Space implies distance, and distance implies travel, which implies a set amount of time. Controlling this time would be key to bringing players together much more often, and we will remove risk of their creations being destroyed (automated respawn, at some sort of liquidity cost, which players will be ultimately competing for by chasing objectives that the game already allows - mining and transporting mass).

I want there to be a real functioning economy, where dumping a load of ore on the market has real repercussions, like EVE Online. If you see the price of iron go down fast, you'll know this has happened. So a player gets a huge cash payout, even though they are selling far below market rates because they wants to "get rid" of bulk iron (which costs them energy and time, of either him or his bots). Because of this event, you as a market participant can buy that up and the reduced price, and have a cheaper source of iron that takes you less time (transport). AI agents will also respond to these events in varying degrees of effectiveness, in order to create competition that economically active players must consider. We want the higher level players to have production assets and ownership in the economy, whereas newcomers can choose to pirate, pillage, or mercenary contracts which rewards them with liquidity if they succeed, which results in them being able to buy equipment quickly and return to combat, but other players may be funding this behavior by selling the ships and weapons produced with ore and energy.

I said big picture because all of these things would come in time only after some sort of standardized transport system was put into place, which the game needs now (it currently uses a simple jump drive which makes you suddenly appear in a new spot) - if we allow extremely fast travel speeds there cannot be collisions otherwise the SIM speed will drop.

Since there are so many physics elements, this game feels like a canvas and I'd eventually want to implement concepts to suggest new technologies that actually may be up and coming in real life. Think of it as a sort of education platform in that regard, but when you don't want to learn, you can just go and realistically blow up space ships and compete on a leaderboard.