I'm really looking forward to this contest, regardless of the specific theme. But I think it would be cool if the theme led to easily-identifiable levels, so that you could take one look at some point in the future and think, "oh yeah, this was for that level contest."
Here's an idea I had in IRC: make a level out of a set number of terrain pieces, say five. They can be any five terrain pieces, but already that severely constrains a level. I don't think it would be too hard to come up with a puzzle, and it's a very approachable challenge for potential participants.