Before we begin, this is all speculation and is in no way confirmed by Disney, Marvel, or Fox.
What It Could Mean For The MCU
The Fantastic Four, commonly known as Marvel's "First Family" is quite possibly the most unlucky comic book team to ever exist. With now two (three if you count the 1994 crapshoot to keep the rights that you can read more about here) failed attempts at bringing the beloved franchise to the big screen, the time is better than ever to bring the team back to Marvel and the MCU. And of course what most fans are wishing for, if Fox and Marvel can strike a deal on the Fantastic Four, then maybe that could also open the door for the X-Men and Deadpool.
Why It Could Work
After the complete and utter failure that was Fantastic Four, executives at both Marvel and Fox are likely trying to figure out what they could do with this seemingly cursed property. Fox doesn't want to waste any more money trying to get this movie to work, and I'm sure Marvel and Disney don't want them to either. However, what I am proposing here isn't a complete recasting, in fact one of the worst things about this movie was that it had a great cast, but still couldn't deliver. Bringing back Miles Teller, Kate Mara, Michael B. Jordan, and Jamie Bell could be one of the craziest - but smartest - moves possible for the two companies.
Why It Might Actually Happen
According to the IMDB for the Black Panther movie, Michael B. Jordan is already cast. Now, this isn't really a huge surprise to many as he is a phenomenal actor, and it was also announced that the film would feature a "90% African-American cast" according to Kevin Feige. Of course he could be playing another character, and that's likely what is going to happen, but I like to think outside of the box, so that's why I think it could actually be a clue for the Fantastic Four to appear in Black Panther. Historically in the comics, the Fantastic Four and Black Panther have been very close, working together on multiple occasions. At this point most readers would likely be heading straight for the comment section to tell me how I'm wrong and how we would know if Marvel and Fox struck a deal, but what if they already did. Of course as I said before it's incredibly unlikely, but still possible, here's why. Simon Kinberg has already expressed interest in making another, brighter, Fantastic Four film. However, with the date already axed from Fox's lineup, that future may look bleak, but if it moved to the MCU, it may not. Fantastic Four was surprisingly dark, especially for that property, but the MCU is typically much brighter, the characters would fit right in, and with Michael B. Jordan already cast in Black Panther, perhaps it's the executives giving us a small clue that the Fantastic Four is coming to the MCU.
Why They Wouldn't Announce it... Yet
With a flop as big as Fantastic Four, there are bound to be haters everywhere and people that want absolutely nothing to do with the franchise and would rather see it just rest in peace. But you can't profit from an idling property, and that's exactly what would happen if Fox just left the Fantastic Four alone. But, if the studios were to announce a deal bringing the Fantastic Four back to Marvel, similar to the deal made with Sony and Spider-Man, they would receive massive backlash, and maybe even boycotts, from angry fans who want no more of the Fantastic Four, especially the failed 2015 reboot. Which is why keeping it quiet is more beneficial. If the studios don't announce a plan to bring the Fantastic Four to the MCU, the fans don't get mad, there is no backlash, and when they see it in theaters it gives fans all the more reason to tell their friends and write positive reviews, assuming it's done right.
As I said this is just an idea, nothing official as of yet, just an observation I had.