This is an off-the-wall suggestion, but the Frightful Four would make a great team of villains for the Fantastic Four to face. They're a great band of lunatic schemers, no matter who is on the team. The Wizard has to lead them, of course, but there are many other members to choose from to fill out the roster. Thundra is one of my favorite villains/anti-heroes, so I'd love to see her appear. Throw in Sandman and that crazy-ass Blastaar and it's a party. Such a shame about poor Medusa, though. Shaved bald and stuck in a bad TV show.