i suggest a multiplayer game, why? because they are the only indie games that really survive for any amount of time, i suggest games similar like what torque does... some simple arena based or level based games with about 8 players, we could make a very exciting multiplayer tank game with just a terrain, few buildings and 8 players, something that could be worth playing, am not fond of aiding any singleplayer game, not that its not possible but it gets stale real quick...

so my ideas:
-8 player tank game [terrain, block based buildings, items, powerups]
-8 player plane game [same as above]
-8 player tps/fps shooter, doesnt have to be a clone... it could be unreal style and use open doors level.. so imagine:

8 players, terrain, few buildings, few fast paced weapons simple movement and animation, fast paced fun, take a look at legions made with torqe, am sure we can EASILY get that quality