In the last five years, we've seen a bit of movement in the villain groups, characters becoming crossover villains from one hero to another. Norman Osborn is one example, moving from a Spidey villain to a full-on Avengers villain.
What Crossovers would you like to see? What villains would be great threats if put up against another hero?
Doc Rot as a villian for various inhuman/supernatural sorts, like Morbius, Werewolf By Night, N'Katnu the Living Mummy
Mr. X as a Daredevil villain (the fights woud be brutal and amazing!)
Kraven as a Namor villain (but the poor guy would have to have some kind of boost to actually survive)
Mister Sinister as a Fantastic Four villain