The Fantastic Four joining the MCU isn’t as simple as producer Kevin Feige simply deciding to make a movie with the First Family.
The rights to the characters have been with Fox since the 1990s and as long as they keep making movies involving the group, will remain there indefinitely.
Unlike other characters Marvel doesn’t have the rights to, the Fantastic Four would be rather easy to integrate into the world as it is; their powers come from intergalactic travel (the specifics vary between stories), which is logical within the MCU as currently defined.
After the failure of Josh Trank’s Fantastic Four in 2015, however, many have been calling for the rights to revert to Marvel (a la Daredevil) or be shared (as Sony are doing with Spider-Man) to allow Reed Richards and co get a definitive MCU take.