11. Stephen Moyer - Mastermind
With ageing actors likely to either be replaced or retired entirely from the Avengers cast (or moved to a less active role, as you have to suspect Tony Stark will be) it's likely fairly soon that Marvel will rethink the Avengers line up, possibly opening up the chance for the likes of Luke Cage and Spider-Man (should rights be worked out) to join. With a New Avengers storyline established, Mastermind would be a significant enough threat to warrant a team-up, and who is exactly the kind of charismatic villain future Marvel films will need to turn to. True Blood star Stephen Moyer would offer the appropriate charm and poise, and is the right sort of age to carry off the character. It isn't enough to just be villainous, there has to be the foundation of charisma that adds complexity and engagement to the best villains, and there's something in Moyer's take on Southern spirit that would work wonderfully.
Who It Definitely Shouldn't Be: Joe Manganiello - despite looking the part, the issue here is that Marvel need to cast him as Wolverine if and when Hugh Jackman hangs up his claws.