It's been assumed that if the Disney 20th Century Fox deal goes through that Marvel would reclaim the X-Men and the Fantastic Four. It turns out only one of those is completely true.
If the monumental deal crosses the finish line Marvel and Disney will re-reacquire the X-Men, but for the Fantastic Four things get a bit more complicated. While Fox owns the distribution rights for Fantastic Four, it only owns part of the movie rights. The other part is owned by Constantin Film.
It all has to do with a deal Marvel made back in 1986. Marvel found a buyer for the Fantastic Four movie rights in Constantin Film, a German production company. In the years since the company has handled production on Fantastic Four, including the infamous Roger Corman film that never saw the light of day in theaters. Constantin made that film solely to keep the movie rights, but Marvel extended their rights back in 1992.
That changed in 1999 when Constantin entered into an agreement with 20th Century Fox, which saw the two companies share the movie rights.
So, if the Disney and 20th Century Fox deal does go through, Marvel will get the X-Men in their entirety, but the Fantastic Four is a little trickier. It could very well be that any contracts Fox has in place would carry over to their new home Disney, so Disney could immediately become the exclusive distributor for Fantastic Four projects.
As for Constantin, the Fantastic Four license has taken somewhat of a beating in recent years, with one franchise that sputtered out and another one that bombed after one film. It would probably be in Constantin's best interest to work with Disney on reinvigorating the franchise (similar to what Sony did with Spider-Man: Homecoming) if they plan on keeping it. If they don't want to be in the Fantastic Four businesses any longer they could just sell it to Disney and move on.
Either way, while it might require a few more hoops, the odds of having the Fantastic Four in the MCU looks more like a possibility than it ever has before.