There had been reports, around the time Fox brought on comics writer and Kick-Ass creator Mark Millar to serve as the "Joss Whedon" of their comics franchises, that the next handful of Marvel properties being developed at Fox would share a universe together -- but according to an anonymous source close to the Fantastic Four production, apparently that isn't the case, at least at present.
"According to a well placed source of ours close to the production of FF, Fox has NO PLANS to cross the universes together," said a report at Schmoes Know (via CBM). "According to our source, Fox REALLY wants FANTASTIC FOUR to stand on its own without having to bring in some X-Men to help the movie along. Our source confirms there’s not even a whisper of this happening because, as mentioned, Fox wants FF to do well enough to warrant another sequel and then another and so on and so forth."
The Schmoes track the progress of the rumors -- after Mark Millar expressed excitement about the properties sharing a universe, the media took it to mean that there would be a crossover film eventually. Millar later clarified his remarks via Twitter, but by then the story had taken on a life of its own and anonymous sources were corroborating it.
With the future of the Fantastic Four franchise up in the air for years now, and a movie finally about to start production that doesn't have a ton of fan support thus far, it may be that an X-Men crossover had to be held in reserve as a Plan B to retain the rights and rehab the franchise should the reboot not take.