I think an
Enemy Territory mod would be very funny. ET is a multiplayer FPS in which the teams have to defend or destroy mission objectives, or one team has to capture gold or documents. There are several player classes like medics and engineers, and they get experience for doing mission objectives, healing, etc. When you gain enough XP you level up and unlock some special abilities or get more ammo.
This is only a suggestion, I have no programming skills, but could help with artwork if someone is interested.
BTW ET uses a modified Quake 3 engine. Maybe we could just reuse the existing maps (there are lots of maps) for an UFO: ET mod.