After viewing daverave's Ocean Scene thread, I think an underwater challenge would be great.

I am unsure on the particulars, but the technical challenges an underwater scene brings would be really challenging We could have it based around a particular type of fish, or each person could have a different underwater environment. For example somebody has in the amazon another the great barrier reef...?

Either that or a Street scene, where you have to model, light, texture and render a street...