This is pure rumor for the moment, but it also makes a whole lot of sense.
When Marvel Studios' ambitious approach paid off handsomely with The Avengers, every studio with a piece of the comic book pie got hot on crossover franchises. Warner Bros. is using the 2015 Man of Steel sequel as a springboard to a whole DC universe, while Sony Pictures is spinning off The Amazing Spider-Man with Venom and Sinister Six.
Fox, meanwhile, is taking a page from the team-up playbook with the upcoming X-Men: Days of Future Past, which combines the continuities and casts of the standard X-Men movies and the prequel X-Men: First Class. It's also rejiggering Marvel's First Family with Fantastic Four, a reboot from Chronicle director Josh Trank.
According to investment site The Motley Fool, Fox could very well pit these two teams against each other in Fantastic Four vs. X-Men. Quoth the site, "After multiple movies in their respective franchises, Fox may decide to combine the Fantastic Four and the X-Men for an Avengers-style movie that could pay off huge for shareholders."
Earlier this month, we learned that the studio has tasked Simon Kinberg with building a shared universe between franchises. Kinberg, who previously contributed to X-Men: The Last Stand, is a writer on X-Men: Days of Future Past and X-Men: Apocalypse, a sequel currently scheduled to arrive in May of 2016. Not only that, but Kinberg is also the latest scribe to work on the Fantastic Four reboot, following in the footsteps of Jeremy Slater, Seth Grahame-Smith, Michael Green, and T.S. Nolan.
"I have a lot of ideas on how to build those brands and do what everybody is thinking of these days: Be like Marvel," Kinberg said not a month ago. "I want to be able to build stories over multiple movies."
X-Men: Days of Future Past arrives on May 23, 2014, with Fantastic Four scheduled to hit on June 19, 2015. Then we've got X-Men: Apocalypse coming May 27, 2016. So don't expect a possible X-Men/Fantastic Four team-up until around 2017 at the earliest.