The Marvel Cinematic Universe seems poised to change in a few fundamental ways over the course of the next few years. Aside from the fact that the world-shifting Avengers: Infinity War will premiere on May 4, 2018, the $52 billion Disney-Fox deal could see some massive developments take place as previously unavailable Marvel properties gradually become incorporated into the MCU.
From there, the fourth Avengers movie is where things could really get interesting. Ahead of the film's May 3, 2019 release, we have put together a list of ways in which some of Fox's most popular Marvel properties could transition into the Marvel Cinematic Universe when the battle against Thanos finally comes to a head. It's not guaranteed to happen or anything, but it's still worth diving in and discussing how Marvel's First Family could be affected by this transition.
Fantastic Four - They Purchased Avengers Tower
In Spider-Man: Homecoming we learned that Tony Stark decided to sell Avengers Tower in order to permanently relocate Earth's mightiest heroes to the upstate New York installation. The buyer of the tower is never made clear, but given the amount of money necessary to purchase something as lavish as Avengers Tower, we have to assume that it is a group with resources. In that regard, it would make perfect sense to learn that Reed Richards and the other three members of the Fantastic Four actually purchased Avengers Tower for the purpose of turning it into the Baxter Building sometime during Phase 4. Who knows, maybe we will even see the cosmic storm that turns them into heroes sometime between now and the release of Avengers 4 in 2019?
Deadpool - Fourth Wall Break
If you know anything about Ryan Reynolds' perpetually smartass Merc with the Mouth, then you likely know that he's a master of the fourth wall break. Although we haven't really seen anything like that in the Marvel Cinematic Universe over the course of the last decade, there's no reason not to assume that Wade Wilson could just walk onto the screen in an MCU-based (and possibly still R-rated) Deadpool movie and simply acknowledge the transition from one studio to another. He did the same thing by remarking on the fact that he finally got his own solo film in 2016 (along with a few other barbs thrown at X-Men Origins: Wolverine) so it would be entirely within character for him to do it next to characters like Spider-Man and Iron Man.
Cable/X-23 - Time Travel
In the world of hard sci-fi, time travel has consistently stood out as one of the best storytelling mulligans available. The X-Men franchise already proved that with 2014's release of Days of Future Past and the same trope could help Cable and X-23 make their way to the Marvel Cinematic Universe from the future. Need an easy way to explain why we have not seen them in the MCU until now? That's easy; state that they haven't been seen by people like Steve Rogers or Tony Stark because they haven't been hanging out in this timeline. All Cable would need to do is bring Laura with him on a journey to the prime Marvel Cinematic Universe timeline and explain that they're finally here and ready to get in on the action alongside the other heroes. Not so hard? Right?
The X-Men - They Come Out Of Hiding
One of the keys to Marvel's success in recent years has been the process of introducing heroes as if they already exist without relying heavily on origin stories. We saw this happen in Captain America: Civil War with the introductions of Black Panther and Spider-Man as two relatively-formed characters. Though it's unclear how Marvel Studios intends to deal with the current continuity of X-Men characters who will show up in X-Men: Dark Phoenix, if a reboot is on the horizon, then Avengers 4 could have the team come out of hiding from their Westchester, New York school and join the Marvel Cinematic Universe. Just act as if they have always been there, but now they are choosing to step out of the shadows and join the other MCU heroes.
Galactus - A Credits Tease For Phase 4
Remember at the end of The Avengers when Thanos turned around to smile at the audience with the promise that he would spend Phase 2 and Phase 3 on a mission to "court death?" That was one of the greatest fan service moments in the entire Marvel Cinematic Universe, and the end of Avengers 4 could pave the way for a similar Galactus to set the cosmic entity up as the big bad for Phase 4. As far as Marvel rogues go, there aren't many villains who can realistically follow Thanos in a way that ramps up the scale of the MCU, so teasing a long-term plan for the malevolent, planet-eating force could help the folks at Marvel Studios offer a glimpse at the next stage in their schedule.
All of Them - Thanos Or Scarlet Witch Rewrite Reality
It's no secret that Avengers: Infinity War is going to be a crowded movie, but two specific characters stand out for their immensely powerful abilities: Thanos with the Infinity Stones and Wanda "Scarlet Witch" Maximoff. Given the fact that both of them will essentially be omnipotent when they reach the limits of their powers, it's not outside the realm of possibility to assume that either of them could potentially rewrite reality in any way that they see fit. Amid all of the storytelling possibilities, this includes the potential to simply rewrite the very fabric of the Marvel Cinematic Universe's reality in such a way that X-Men, the Fantastic Four, and all of the other Fox-owned Marvel characters have just always been there.