There's a huge rumor that is spreading across the Internet right now that 20th Century Fox might be developing a Fantastic Four Vs. X-Men movie. If it's true the movie would be based on the 1987 miniseries Fantastic Four vs. X-Men by Chris Claremont.
After multiple movies in their respective franchises, Fox has now decided to combine the Fantastic Four and the X-Men for an “Avengers”-style movie that could pay off huge for shareholders.
Based on a 1987 four issue comic called Fantastic Four vs. X-Men, the movie will see the characters against each other because of secrets regarding the Fantastic Four’s origin. The new movie also comes as Fox is rebooting the Fantastic Four series with new actors and actresses.
If this movie does happen it probably won't happen for a few more years. It definitely wouldn't happen until after the Fantastic Four reboot being developed and Bryan Singer's X-Men: Apocalypse, which doesn't come out until 2016.
There's nothing official about this right now, but it doesn't seem like a too far-fetched idea. All these films in development could all be leading up to the crossover. If this does happen, we probably won't see it until 2018.
Here's a detailed plot summary of the comic from Wikipedia, although I'm sure things will be adjusted and changed for the film.
Franklin Richards has a nightmare, wherein his father, Reed Richards, murders both the Fantastic Four and the X-Men. Reed then takes a book, disregarding the protests of his son, who says that the book is the cause of everything bad that happened. Reed opens the book and it transforms him into Doctor Doom. Waking in terror, Franklin finds his parents and tells his mother about the dream. As Susan unpacks boxes, she finds Reed's college journal -- which Franklin recognizes as the book from his dream. Ignoring Franklin's warning, she shows the journal to Reed.
At Muir Island, the X-Men and geneticist Moira MacTaggert are in dismay. Kitty Pryde's phasing abilities have gone awry, making the atoms of her body drift farther apart. Magneto suggests seeking assistance from Reed Richards. Traveling to Four Freedoms Plaza in New York City, Magneto persuades the Fantastic Four to help.
Arriving on Muir Island, Reed seems troubled. After examining Kitty, Reed states that his molecular reintegrator device cannot help her in her advanced condition. Magneto, having read up on the device's capabilities, contends that Reed is lying, prompting Wolverine and Rogue to attack the Fantastic Four in order to force them to help. X-Men leader Storm diffuses the situation and renews her plea, but Reed still refuses. The teams are interrupted by a holographic communication from Doctor Doom. Doom offers to duplicate Reed's research, reconstruct the molecular reintegrator and save Kitty. Reed protests that they should not indebt themselves to Doom, but Storm counters that Reed has left them no choice.
Later, Susan reveals the contents of Reed's journal to the rest of the Fantastic Four. A key passage of the journal states that Reed planned to counter the potential supervillain threat by flying Ben Grimm, Johnny Storm, Susan and himself into the cosmic rays which would turn them into the Fantastic Four. Although Reed denies writing the inflammatory passage, Ben, Johnny and the She-Hulk leave Four Freedoms Plaza.
In Latveria, Kitty prepares to commit suicide in order to prevent the X-Men from indebting themselves to Doom for her sake. Franklin, present on the scene via one of his "dream-selves", successfully pleads for Kitty not to do so. While Reed wonders if he might have subconsciously planned for the cosmic radiation to bombard the rocket years earlier, Sue eventually realizes that Reed could not have written the passage from the journal, citing how he responds to Franklin as a parent where the clinical man in the journal would never have done such a thing. Feeling compelled to overcome his self-doubt and help Kitty, decides to travel to Latveria (with Franklin in tow). The other members of the Fantastic Four, having come to terms with the changes their powers made to their lives, return to FF headquarters and volunteer to accompany him. Misunderstanding their intentions, the X-Men attack the Fantastic Four upon their arrival. The two teams relent when Franklin arrives and reminds them that saving Kitty is what is important.
Doctor Doom begins the treatment to save Kitty, with Reed overseeing. When Reed claims that he has made a crucial mistake in the reintegration process, Doom refuses to listen, but Franklin speaks up and shames him into letting Reed help. Seeing Doom's reaction to Reed's struggles with the process, Susan realized that he forged the journal and planted it among Reed's things years ago. Reed successfully navigates the problems with the reintegration, saving Kitty's life and causing her molecular structure to begin its slow return to normal. While the Fantastic Four and the X-Men make amends, Susan confronts Doom, but he will not admit to having written the journal entry. After issuing a veiled threat to Doom, Susan allows the celebration of Kitty's recovery to resume.