Marvel has a deep library of characters to use for the MCU, but there are two properties that the company still can't pull heroes and villains from for its movies and TV shows (with rare exceptions): the X-Men and the Fantastic Four. Both properties still reside at 20th Century Fox, and given how the X-Men franchise is still experiencing varying degrees of success, it's extremely unlikely the mutants will be added to the MCU. However, there's been continued discussions among fans about if Marvel and Fox might work out a deal to add the Fantastic Four into the MCU, but according to Marvel Studios president Kevin Feige, there aren't any talks between either companies about making that happen. When asked if there's been any kind of "thaw" or "talk" between Marvel and Fox regarding the Fantastic Four, Feige said:
No, but I would say there's not like there's anything frozen that there needs to be a thaw. There's no weirdness. It is just very very cut and dry, and they're doing their thing, and they're doing it quite well in most regards, and we're doing our thing.
For those of you who've been wondering if Marvel and Fox might partner up with the Fantastic Four like Marvel and Sony did with Spider-Man, as of right now, that's not in the cards. While Kevin Feige assured Collider that there isn't anything weird going on between the two studios, there isn't activity on incorporating the Fantastic Four into the MCU. Marvel and Fox are working on their respective projects, and for right now, that's fine for both sides.
It's still unclear what Fox future plans for the Fantastic Four are. The 2015 reboot was supposed to kick off a new series, but after bombing critically and commercially, the sequel set for summer 2017 was taken off the schedule, and there's been no indication that it will be revisited. The closest thing to an update came last February, when producer Simon Kinberg said that one of the lessons the studio learned from the reboot is that they need to make sure the next Fantastic Four movie is "100% right" before moving forward since they would "not get another chance with the fans." That implies Fox is still keen on staying in the Fantastic Four business. Still, considering that the studio's previous Fantastic Four movies failed to critically impress, one would think the higher-ups at Fox have at least considered chatting Marvel up and coming to some kind of an arrangement on the superhero team. Alas, nothing's happening on that front...for now.