I wouldn't mind seeing the Masters of Evil. Granted it'd probably be better if they set up the Masters like they did the Avengers -- a bunch of villains intro'd over several films -- but that'd probably be difficult, especially since they'd just about have to start from scratch since most of the villains so far have gotten bumped off at the end of each film.
...That said, it wouldn't be hard to bring back the Red Skull and then throw in villains like Hyde or Absorbing Man in the Cap and Thor sequels and then toss them all in to Avengers 2. Avengers was the first real Superhero team-up, so it seems the only way to top that would be with the first supervillain team-up. Or something like that.
Have a good day.
G Morrow