An MMO shooter with permadeath where players form massive mega-corporations and higher-ups gain the ability to order lower-level players around (by changing the quests that their questgivers hand out). Players that teamkill are blackbarred from their guild/clan/corporation and become free game, so there's quite a bit of incentive to be a good little drone and follow orders. However, massive revolts can also become a thing, as well as backstabbing and assassination on the higher levels. All of this is ordered by players, to be executed by players. So sort of like EVE Online, except as an FPS.
Sounds like SF FPS Battlemaster
On that topic, I want an FPS set entirely in zero-/microgravity. It would be sci-fi themed for obvious reasons. The movement scheme would be very... original, and there would be a big learning curve, based around pushing off or holding on to various objects, jetting around, and just outright drifting.
Also, view/turning controls would be a bit different - your view wouldn't maintain a constant roll and pitch like it does in most FPSs, and you would have to carefully adjust it if you wanted to face the right way. Rotation on all three axes would be possible - but you could only turn so far (i.e. turning the head) before you would have to use thrusters or grab hold of something. To keep turning the camera from getting
too difficult to control (i.e., spinning forever and requiring manual adjustment), there would be a control to automatically engage retrothrusters to help stabilize it (futuristic computer technology or something). Needless to say, though, this would not be game for the motion-sickness-prone.
The main method of maneuvering would be the jetpack (think those old NASA things). It would be built into whatever protective suit the player wore. As mentioned above, this jetpack would be sophisticated enough to allow turning on three axes, plus moving (from the player's point of view, of course) "up", "forward", and "sideways" (probably not down or backward, although there could be multiple versions of the jetpack with their own strengths and weaknesses, one or two of which would have such movement as special abilities). The jetpack would also have limited (but replenishable) fuel, however, which would partly incentivize grabbing and holding objects, as well as the fact that the jetpack would be a bit finicky to control compared with just holding a rail and clambering along.
Honestly, any first-person game would work (vehicular, for example - you could even work space vehicles into this). Hell, the combat would probably be secondary to the maneuvering. I just thought FPS because I've seen spaceship sims in the past - and because I got the idea while crudely simulating a jetpack in HL1 via cheat-commands, keybindings and macros. Flying through Black Mesa like you're in space with a jetpack turns out to be wildly fun.