Just some ideas:
Movie characters: must clearly identify the character you're portraying and the movie said character appeared in.
Book characters: same as with movies, but draw inspiration from your favorite book.
Fairy Tale: there's a host of fairy tales you could draw from. Again, clearly identify which fairy tale you're portraying.
We've been doing seasonal and holiday contests for so long, I'm just trying to think of other sorts of things we could do.