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I think this deal has been in talks for a little longer than the public is aware. Marvel has been making some moves as of last year to set up something called "The Multiverse" in the MCU. Some of these Fox films and shows are very much still in development or nearing completion. They are not going to let that go to waste.
- The Fantastic Four is probably the primary key IP that will be capitalized in the mainline MCU.
- Bob Iger confirmed that Deadpool will remain R rated. This leads room for other Marvel films to do the same.
- The mainline MCU has InHuman's and enhanced individuals. Mutants has been written out and the MCU is already filled with Mutants that are now InHuman's and the InHuman's presence in the MCU is undeniably large. X-Men will be part of the Multiverse that has already been deeply set up, versed, and explained to people in Dr. Strange along with Agent's of S.H.I.E.L.D. Season 4 and 5.
- Jeph Loeb, the head of Marvel TV and I believe he has pull in the comic division (Marvel's TV and Comics division are closer than the film and there's more ties but the TV stuff is still part of the MCU) has worked with Fox on their TV's shows being Legion and The Gifted. Legion is a good starting point to adapt to the MCU as they are working on season 2. The quality of that show is absolutely phenomenal. I plan to watch The Gifted as well, but it won't connect very well thanks to the use of the Strucker bloodline which already exists in the MCU with Baron Wolfgang Von Strucker and his son Werner Von Strucker.
- Rumors circulated last year that Latveria was spotted in Infinity War. I have a lot of doubt for this but it could be possible some deals were made in the past.
- X-Men could very well just exist as it's own thing outside of the MCU.
