I would like to see them face off against a heavyweight as opposed to some of the more obscure villains, mostly because when they do come back they will need the publicity.
Doom (Though I think he would put the hurt on OF, mostly because marvel wouldn't want to let Doom get jobbed like the Wrecking Crew did)
Sinister and the Marauders (A big motley crew of mutants VS a motley crew of mystic heroes? Could be fun)
The Thunderbolts (Might be good considering that they are managed by Shield and the Initiative. Maybe they chase a fleeing hero all the way to the border and fighting ensues.)
Weapon X leftovers with Jeffries (They had a lot going in Canada, and with the SHRA and the Initiative, they would not be too welcomed performing their experiments in the US)
The Juggernaught BIATCH!