As of right now, we're 99.9% sure that the Fantastic Four and X-Men will arrive in the Marvel Cinematic Universe at some point in the next few years; it ultimately boils down to whether or not that Disney/Fox deal goes through without a hitch. Regardless, Kevin Feige will have his work cut out for him introducing the heroes into an already established world but he'll no doubt find a way to pull it off.
During a recent interview, Joe and Anthony Russo were asked about how best to introduce the two teams into the MCU and while they chose their words carefully, it sounds like they have some ideas.
In fact, they pointed to Secret Wars - a series which brought together a number of alternate realities and mashed them all together during its latest iteration - as being one possible solution for that.
"If Disney's takeover of Fox goes through, there are a lot more characters to work with all of a sudden, it might be interesting to do something like Secret Wars," the filmmakers teased before moving on, presumably in a bid to avoid potentially giving away anything Feige might actually be planning. Bear in mind that it's possible the Russos were referring to the original Secret Wars event.
In that, heroes from all over the Marvel Universe were pitted against a number of villains at the behest of The Beyonder on a planet created solely for them all to fight for his amusement. Either possibility would be fun to see, of course, but it's the former which could best explain the arrival of the X-Men...