For a co-op game, how about this idea?

You awake and find yourself and your teammates in a kind of control room. They tell you that they seem to be an a abandoned space ship which was lost during the last big war 200 years ago.

Your mission/sub missions:
- find the master control room to enable the emergency energy
- find the systems video log, to understand why this ship is abandoned and what kind of problems might be in your way
- disable the autonom intruder detection systems on your way
- find the "energy crystals" (bio gel packs/nuclear battery) to power the lower space ship section
...

This way we could introduce several puzzles, autonom security bots, ...

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This game could be played as
- single player with npc team mates
- hot seat (switch between persons)
- LAN mode (without chat system)
- split screen (two cameras) using joysticks for input

EDIT
Currently I try to make a deal for a texture + model pack for such a game. This could give this project a big boost