As exciting as a McAvoy/Stewart team-up would be, I'm far more interested in seeing what happens when you put Michael Fassbender and Ian McKellen together as two faces of Magneto.  Fassbender's portrayal of the character in "X-Men: First Class" gave a fresh face to the simmering anger that was always subtext in McKellen's work.  Fassbender is far more physical, far more emotionally raw, and while McKellen always played Magneto with a very focused sense of purpose, coming face to face with his own youthful fury might shake him up and rekindle his own desire to establish mutantkind as the dominant force on Earth.  These two together might be impossible to stop.

- Drew McWeeny