Custom theme contest. Make a map in a theme unused by any official maps. Deserts are boring.
Yep, the percentage of desert-themed maps is way too high.
However, I'd say a contest based on existing themes would probably work out better.
I mean, not even valve can make new themes (like Soho , or Grordbort items) without half the community complaining about "breaking the art style". Custom themes would probably go the way of Hydro and Junction...how many commonly played maps have the swamp theme, for example?
But why not make something out of the themes we already got?
Halloween, Soho, and Spacecraft/spytech are all rather rare.