Many things, but I mostly need:

-Textures w/ or w/o normal maps. I currently use the standard.wad textures, which have the worst resolution in the world and are used everywhere. Any textures are fine: this game doesn't have much of a theme. Whatever is cool, feel free to do it.

-Models of anything: furniture, flora, monsters, weapons, space marines, you name it. Once again: whatever's cool, do it.

-Maps: space stations, moon bases, deserted planets, ancient otherworldly ruins, and anything else you can think of. The game will be like UT in that many of the maps won't be related to one another. However, I should note that the game will be on a small scale with a 4 player limit (yes, 4 players: this isn't going to be an MMOFPS), thus the game will be great for small LAN parties and stuff.

I should take the time to note that the cooperative gamemode will probably be like the the Unreal game mode "Invasion": play on a multiplayer map, fighting off large amounts of alien enemies with your friends until the time expires or something.