User blog comment:Rockman1216/The Fox Marvel dilemma/@comment-993964-20160105234835/@comment-9062114-20160106010450

Fox doesn't need to sell the X-Men rights back to Marvel, they can keep the rights exactly as they have it now like Sony does with Spider-Man, they would just allow the MCU to have the X-Men and use it's characters, concepts, locations, etc (while the X-Men movies would in turn be able to use other things from the MCU).

Fox would still get to make X-Men movies and still profit off of them, except they would have the ability to use the MCU's vast universe for content in them. The X-Men play a big part in the Marvel Universe and if Fox wants to get into the Cinematic Universe business, they might need other properties to properly do the stories and characters they have well!