I think it might be good to narrow it to one or two themes also, and I'm betting we could supply some reference images as a starting point for people (they wouldn't have to use them, and it's not about copying an image, but rather emulating a theme).
One thing that might be fun, is to pick one theme that combines two contrasting aspects.
For example, I think it would be cool to build a vista with the theme
Industrial vs Natural. This lends itself to the halflife 'look', which is a good start for people and wouldn't require people to make alot of textures and models. Another example would be
Ancient vs Futuristic. With these two, I can already picture a scene of ramshackled industrial huts, taken over by growth on a rocky cliff_side, over looking a river or a futuristing combine like structure built upon the fallen stone of old Mayan ruins.
Sounds cool to me