Some of this is based off other articles I read, news I've seen, and my underdeveloped mind...
As we know, Fox owns the movie rights to X-men. What Fox doesn't own is the TV rights. At about the same time Fox was battling for the TV rights, FF flopped big time. This flop gave Feige a spider-man-like opportunity to swoop in and either get back the FF property or share it.
Fox only has X-Men if it lets go of the FF so it is still unlikely that Fox is so willing to give up FF. However, X-Men has been an incredibly lucrative property for Fox. Think about not only X-Men movies, but deadpool, wolverine, and the potential for so many other characters to make spin-off movies.
For some reason, right after FF flopped, Marvel (which has a terrible terrible relationship with Fox) gave Fox the rights to make X-men tv shows (two are on the way, Hellfire Club and Legion). Why in the world would Marvel give something away to a studio it hates and get nothing in return? Feige would never do that.
Absurd leap #1: Fox and Marvel made a deal for the FF and ultimately, it's expanded universe of characters, which mainly include Silver Surfer and Galactus. Incredibly, both studios are keeping this a secret.
Absurd leap #2: Marvel has announced 3 untitled movies in 2020....could one of those be the Fantastic Four? Could one be the Silver Surfer
Absurd leap #3: Feige has just announced that Inhumans is being delayed. Feige claims that things come up (which I believe) and cited the new Spider-Man arrangement and new Spider-Man movie as one of the reasons. However, what if Inhumans is being held back because there is now a better way to expand the cosmic universe? What if now, they have the Silver Surfer, a more recognizable character than any Inhuman (to the general public). What if they now have Galactus as the next main villain, after Thanos (stole this idea from moviepilot.com)? Wouldn't that be a logical step for a shared universe when you neeed to find a villain that can actually match the power, if not surpass the power of Thanos? Why go into an untapped well like the Inhumans when you have more name worthy characters you can use to explore the cosmos? Granted, maybe Silver Surfer left a bad taste in everyone's mouth thanks to Fox, but I think there is a pretty good chance Marvel can reinvigorate the character. Plus, Feige and Isaac Perlmutter aren't the best of friends, and Feige was rumored to not be thrilled with Marvel branching off into TV with the same universe...so could he be pushing off Inhumans as punishment for Perlmutter and his involvement with Agents of Shield (I kind of scooped this idea from a CBM article). But it makes sense that Feige has more than enough motivation to find better and easier ways to dig further into the Cosmos.
And wouldn't an FF, SS or Galactus nugget be an insane one to drop in a post-credit scene in any of the upcoming MCU films?
Again, this is all ridiculous, rumor-filled, unfounded jargon, but the complete radio silence from Fox and Marvel over the past 5 months or so regarding the FF may make you wonder...