My Masters Of Evil Line-up
THE ABOMINATION - The Abomination survived the "Hulk" movie, so no reason why he can't return for a rematch, especially if The Hulk is involved somehow.
THE CRIMSON DYNAMO - It looks like Crimson Dynamo is gonna be in the "Iron Man" sequel, and CD survives, he could join the team out for payback on Iron Man
THE WHIRLWIND - He has history with Henry Pym and Janet van Dyne, so he could be Ant-Man and The Wasp's counterpart. Plus he's been a long-time Avengers villain and he's shown up quite often.
THE VIPER - Every team needs a lady on board, and The Viper is a dangerous member of Captain America's rogues gallery. A psychotic anarchist and not too shabby in the hand-to-hand combat department, she could be Captain America's enemy on the team as well as the classic femme-fatale.
THE ABSORBING MAN - This guy took on Thor and The Hulk by himself, but he's also duked it out with the entire team of Avengers. In terms of sheer power, he'd be a formidable foe (they just have to not make the same mistakes they made with him in Ang Lee's "Hulk" movie). Just make him a subject of genetic manipulation or even a mutant (or if Loki's involved, then you have your "a wizard did it" explanation)
KLAW - I'd only use him if they introduced The Black Panther, since they created each other, more or less. Sonic weapons aren't all that implausible, given the rules these movies are playing by.
As for Ultron, I could see a quick reference or two of him and save him for the sequel, where we have the Vision and stuff.