Great idea, i was thinking of something like this. I would like to make an indoor map with vieuws outside, custom models and textures.
Hmm ideas ideas...
You can choose what type of map you want, but it has to be ctf , cp or tc, so no shit funmaps.
Time; 3 months.
Theme, should be free, you dont want the contestants make all the same map
Creativity, gameplay and looks are important, also, a server should be available for testing the block map before adding detail.
Rewards: the biggest reward for a mapper is that his map gets played. Maybe garantee that the map will be on most server (you post the map on forums, recommend it etc etc, announce the competition big...)