Btw, what's up with people requesting all these copyrighted characters? First Jason, then some character from MLP, now it's the Joker... try to come up with your own ideas, guys, it will be more awesome in the end!
On topic, I'm not sure what I'd like to see... seems like most types of monsters have already been done to death in countless games.
I think I'd like to have an enemy with a personality of sorts. Someone or something (doesn't necessarily need to be an intelligent being) that appears more than once and even though you may not know what exactly it is for a long time, you still know it's the same thing throughout the whole game.
Think of the first Alien movie. One spaceship, one Xenomorph and for the most part of the movie, you don't even know exactly what it looks like or what it's capable of. If that mechanic works, it doesn't even really matter what the monster looks like when you finally see it...