At the London premiere of Avengers, producer Kevin Feige dropped several hints of X-Men characters appearing in future Avenger films. The two mentioned were Quick Silver and the Scarlet Witch, the children of Magneto.
While 20th Century Fox does own the X-Men movie license, characters like the 2 X-Men and Skrulls are some of the few character usable by both Fox and Marvel. This is not confirmed but the possibility still stands and these two universes may finally come together through this.
For the rights issue breakdown, check out this interview