If map from base/layout, I think it would be better to let each individual mapper to choose their own theme. I think it will give much more varied results that show more creativity by how they chose to make their theme fit the layout.
I have a few ideas for compo 'theme':
Map from other media. I know its been done before. Recreate a level from another game or a level based off of a tv show or movie. (Remake SP level from goldeneye (new layout), halo mp map (similar layout, different theme), the megacity from Dredd, that one water village in the Desolation of Smaug (Hobbit 2))
Map from real life location but in an altered state (futuristic, wartorn, post apocalyptic, medieval, alien civilization). Include pics/video of what the original location looks like (if its not a common public place people can google). Example: Eiffel tower as a landmark (or secret alien comm facility) on an alien planet.