For many years, Marvel has had its properties splintered across various studios, but when Spider-Man partially returned to Marvel earlier this year, that left only one real competitor. That competitor was 20th Century Fox, which own The Fantastic Four and the X-Men Series. Now, most people don't care much about Fantastic Four, but many are angry at the separation of the X-men from the rest of the universe, especially since the X-men universe has been called as expansive and big as the surrounding Marvel universe. To add these two universes together would create a film universe almost perfectly mirroring the comicbook one.
Given Fox's and Marvel's plans stop being set in stone around 2018-2019 and have instead become conjecture, many have wondered what their plans past that are. According to a rumour started by 'Pete's Basement' (a comicbook talk show) based on information received from a man by the name of Mikey Sutton, the Fox and Disney/Marvel universes are planning to buddy up for a 2020 film featuring the X-men and The Avengers.
Will this come to pass? How? I'm taking a guess at the Beyonder arriving and making some sort of Battleworld, incorporating elements from both 'Secret Wars' stories, but that's just me.