We know that there is now a very good chance that we will see a sequel to Mathew Vaughn's X-Men: First Class and that the original cast are all set to return. We don't know if the same can be said of Vaughn, but we should find out one way or the other soon enough. But what of the story? Well, the last movie ended with Charles and Erik parting ways and suggested that if we saw them again it would be as fully fledged enemies. But Michael Fassbender reckons that may not be the case, or at least, he wouldn't like it to be. He says the fact that Magneto still has some ambiguity to him, and that Xavier sees it is what interests him most about the characters, and he wants that to continue in the next movie - “Hopefully we’ll get to do another one; that’s what we want to carry that vein through for sure. Personally, I do and I know James feels the same.”.
Here is an excerpt from IFC's article..
Referencing the on-again, off-again friendship between McAvoy’s character, Charles Xavier, and Fassbender’s magnetic mutant in the Marvel Comics universe, Fassbender hinted that there’s always the possibility that they’ll be on the same side again. Although if their comic-book history is any indication, it could take another threat bigger than both of them to bring the former friends together.
“You know what’s interesting about Magneto and Professor X from the comic books as well, is there’s such a complexity to their relationship,” he explained. “It’s not just like clear-cut enemies; they’re best friends as well. In the comic books, even after they’ve had this sort of rift, Professor X asks Magneto to come back and look after the students at certain points.”
“I think there’s always that complexity in their relationship,” he said. “And we want to keep that alive as possible, because that’s I think a really interesting thing — the conflict there.”
So what could make Xavier and Magneto join forces again? Of course even IF that happens we won't know for a long time but speculation is welcome! My money's on Apocalypse. Ok, not really, but a guy can hope right?