Disney purchased Marvel in 2009. However, there were certain characters that were already licensed out to companies like Twentieth Century Fox. This means that as long as these companies use the characters to produce things, mainly movies, that Disney could not have them in the Marvel Cinematic Universe (which contains the Avengers etc.).
Recently, Disney has gone into discussions to buy the entertainment properties of Twentieth Century Fox (after this referred as Fox). Now this may breach anti-trust laws meaning it does not happen. Yet let’s presume that Disney’s buying of Fox is successful. This would mean that the X-Men and Fantastic Four will show up in the Marvel Cinematic Universe (presumably) with none of the history of earlier movies.
Now, there are two ways that these two teams could be added into the MCU. The first is just to introduce them in the same way as Spider-Man has been. While the second way is to retroactively change continuity so they always existed, yet nobody mentioned them, much like Damage Control in Spider-Man: Homecoming. While both methods can work, they each have various downsides.
The downside of simply introducing the X-Men is that fans who love the Fox movies could feel alienated by a do-over. Also, the Inhumans have been given a similar hated and feared status, so in-universe that may cause the problem of too much of the same thing. While retroactively adding them would create the problems of having two Quicksilvers, and very public battles that the Avengers or S.H.I.E.L.D. should have taken part in.
As for the Fantastic Four, I don’t see a downside to just introducing them. However, there is a problem to retroactively introducing them. I mean, how does one not mention, or show reference to celebrities with powers, especially when this is a part of the core of this concept?
In conclusion, while we can cheer at the idea of these properties being incorporated into the MCU in some way, it may not happen immediately due to the downsides.