I would love to see some of the old enemies that didn't make it come back. I realize that's a pretty big work commitment, so doing it as fairly-priced DLC would be totally acceptable.
I particularly miss the Hamsterball (and the "flattened" effect that went with it) as well as the fairy.
Also, I could see the FunkVac coming with that DLC package. Instead of jars full of earthling, it could spit out presents after sucking up all enemies in the area.