Landscapes. Build an epic landscape! (Very broad, sorry.)
Famous historic happenings.
I think those would be cool, but all of them have probably been used in some sort of contest, but you could still re-use them. Permalink
say, from america, statue of liberty, classic muscle car or a cowboy saloon.
anything famous for being from one country - this gives a lot of potential as there are many categories - those who build cars (like me) an stick to cars, those who prefer buildings can do those and so on. Permalink
"Riddly fiddly doldy ho!" Make a creation based off that line. It can be anything you wish, but it has to relate to that line in some way.
(integrate it into the story, maybe?)
"Peaceful space." Build a spacecraft that has a peaceful purpose, and no weaponry or troops.
"Minifig crazyness" Make something (not a minifigure) using lots of minifigure parts. It can be anything you want. but it has to be clearly visible that it has lots of minifigure parts (torsos, heads, arms, legs, hands, ect).