How about picking a serious subject or scene and making it wacky and over the top?
Eg there are tons of pics of Lord of the rings but theyre all deadly serious. It would be fun to see one of say, helms deep, with Gandalf in a frenzy, leaping up and clobbering orcs with a crazed smile on his face like he was enjoying it and all the characters just generally over the top and exagerated, but not neccesarily meaning changing it to a more cartoon like style. just miking it kinda crazy.
I also thought about taking an anime or cartoon character and doing a realistic depiction of them. Not very original though, maybe.