It all depends on the "flavor" of the game... 007: Nightfire (lots of weapons, but all of them are pretty good) vs. Halo (generic names, well balenced) vs. Doom 1 (handful of weapons, porgressive coolness) etc. etc.
For buildings, however, I would suggest putting all of the really cool stuff on the first level, like bungie did with Halo. Like a spinning radar dish, or props that have animted textures
And elevators! Elevators are cool... Of course, you're going to have to do alot of corodnation with the one who's doing all the coding, but such things really add alot to a game
Aside from that, I can't think of anything really... Just use your imagination! Be creative! And have fun!
Bring out the mercenary in you!
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