For a contest I'd love to see something like "create your own theme" where you can work in teams, to create an environment (doesn't need to be much, just some models and textures would be fine but it all depends on how long the contest would last etc) It wouldn't matter if more than one theme created the same theme, and would be judged by popular vote. It would also give people some more experience in creating custom content for maps and things, broaden their horizons, so to speak.