According to IGN on Aug. 19, in a previous interview with Mark Millar there had been talks about possibly crossing the next "Fantastic Four" film reboot with the "X-Men" movie franchise. However, it was later cleared up by Millar via his Twitter feed that it wasn't the case.
"I keep seeing 'news' that X-Men & FF movies are crossing over. No, they're not. They exist in same universe but movies stand alone".
A recent interview revealed that Millar stated the obvious about Marvel always having other characters appearing in their movies and spoke of the possibility of crossing over other characters from different franchises since Marvel and Disney are known for it. He also figured that what brought upon this exciting idea in his thought process is when he first saw Nick Fury appear in the first "Iron Man" film.
It does seem that Fox is open to crossing over the characters' appearances, but not at this time. That being said, until that day comes, most fans should be content with just seeing a plethora of mutants in Bryan Singer's "X-Men: Days of Future Past".