The game still has way too few villains, and that means that every single PVE tends to feature the same villains, namely the Dark Avengers. Since various aspects of the game tend to conspire towards limiting new character designs, one approach may be to pull the SHIELD/HAMMER goon cloning trick on a larger scale, but only for PVEs. Specifically, clone more of the characters ONLY as PVE opponents, not as released characters. Someone had suggested this a while back for character releases, which I was (and am) against, but it might help with PVE variety.
Namely, Wolverine could be cloned as Sabretooth. Hulk as Abomination. Spider-Man as Paste-Pot Pete. Human Torch as Pyro. Etc. All you’d need is new art and new power names. This would give you a big chunk of “new” villains to spice up PVE (a la the Skrull imposters as Iron Man) without it taking up all the design team’s time. And it would mean that we wouldn’t be always fighting the same ten characters in PVE.