Magneto & X
Following the trend of spin-off movies (first starting with the planned Wolverine series I mentioned), it looks like Magneto may be given his shot:

Sheldon's screenplay will show Magneto seeking revenge for the murder of his family in a Nazi concentration camp, watching him discover the extent of his powers, and the start of his friendship with Professor X (played by Patrick Stewart in the X-MEN films.) Xavier was an allied soldier that helped liberate the prisoners in the concentration camps during World War II. After the conclusion of the war Xavier and Magneto meet and become friends, united by their mutant powers that they hide from the public's eye. Eventually their two different perspectives on the rise of the mutants begins crumbling their friendship, turning them into rivals.

Sounds like a decent pitch, but with all movies, the writing will be key. Certainly are enough mutants in the Marvel Universe to warrant a tide of spin-offs; I'm sure that Gambit could warrant his own series without even a mention in X3, whenever the hell that comes out.