The Marvel Universe is owned by three different companies. Although Marvel may put whatever characters they want in their comics, the only company who can use the characters of the Spiderman canon is Sony and the only company that can use the Fantastic Four or X-men is Fox. All others belong to Disney.
I wrote an article about this earlier: https://captainzach616.wordpress.com/2014/02/03/character-rights-who-owns-what-and-why/
All that being said, rumors have arisen about a crossover. Before you check the date, no this is not another April Fools article. Variety has reported that the post-credits scene of The Amazing Spiderman 2 is going to feature Mystique, Havok, Toad, and William Stryker. These characters are owned by Fox, not Sony.
If this is true, this is the first marked crossover between copyrights in Marvel Cinematic history. Variety reports that this is a one-time deal, not the beginning of some shared universe. I would personally doubt that. Once the characters are acknowledged as existing in the same universe, how do you return to pretending they don’t exist?
This could also be an attempt by the two “lesser” companies to challenge Disney, who has been capitalizing the Marvel movies since Iron Man in 2008.
I wonder if this is even true. It could be some rumor started and accidentally believed by a large source. If this is correct, this is some massive news. It could be the beginning of burying the copyright hatchet between companies and striking some deals that could reunite the Marvel Universe. It could open up the door to some major story arcs, such as the Civil War or the X-men vs Avengers.
I’m still taking this with a grain of salt and, as usual, only time will tell.