As excited as fans are to see the X-Men join the MCU, Marvel would be wise to wait until Phase 5 to bring them into the fold. The Marvel Cinematic Universe was a novel approach to blockbuster filmmaking when it was first conceived, one that's resulted in unprecedented levels of critical and commercial success. So far, the 17 entries into Marvel's shared universe sport an average 83% fresh rating on Rotten Tomatoes, and they've combined for a total haul of $13.5 billion at the worldwide box office. Earth's Mightiest Heroes have quickly become Earth's Mightiest Franchise, but even the MCU's most fervent supporters have had trouble ignoring the elephant in the room. Plainly put, a cinematic universe based off of the heroes of Marvel Comics is incomplete without the X-Men. (And, to a lesser extent, the Fantastic Four.)So when Disney/Marvel was finally able to reacquire the film rights to those characters from Fox, fans went nuts. At long last, the MCU has access to Marvel's First Family and the mutants that shaped the comic universe as we know it. The only question that remains is how and when these superhero teams will be introduced.Related: How Long Until X-Men Join Spider-Man and the Avengers?That's where things get a bit messy. Between narrative complications, various regulatory committees, and the litany of projects already well into development on both sides of the aisle, bringing these beloved heroes into the MCU will not be as straightforward as fans might've hoped. It can (and likely will) be done, but it's going to take some patience. Here's why Marvel Needs To Wait Until Phase 5 Before The X-Men Crossover.

The deal hasn't actually been completed yet

X-Men joining the MCU

When the deal between Disney and Fox was "completed" in December, most fans assumed that the X-Men and the Fantastic Four would be jumping into the MCU in short order. Why wait to give fans something they've already been waiting years to see, right? Because while the deal has been agreed to by the two companies, it still hasn't hasn't been approved by the proper regulatory committees. Governments both at home and abroad are expected to submit this agreement to the same sort of scrutiny that the 2016 Time Warner/At&T deal was subjected to (a deal that the U.S. Justice Department is currently suing to block, by the way). TLDR: Marvel's purchase of Fox hasn't actually been completed just yet, and it's not quite a sure thing, either.

Related: Marvel Artist Says X-Men Won't Join MCU Anytime Soon

A lot can go wrong in the 18 months that the regulatory process is expected to take, so the companies have financial agreements in place in the event that one of them ends up backing out of the deal or that it's blocked. But since there's a lengthy road ahead to the deal actually being finalized, the MCU and Fox will continue operating autonomously. Neither company can afford to assume that the deal will be approved, so until it actually is, both Marvel and Fox have to operate as though it's not happening. Marvel Studios president Kevin Feige may be excited to work with the X-Men, and he can (and will) concoct a gameplan for introducing them, but he'll have to wait at least another year and a half to actually do it. Neither company will be sitting on its hands in the meantime, though.

Fox's Marvel movie universe has A LOT going on right now

One of the first questions that sprung to mind in the aftermath of the Marvel acquisition was what the deal meant for all the X-Men projects that Fox already has in development. The studio has a trio of movies that have already wrapped filming -- Deadpool 2, X-Men: Dark Phoenix, and New Mutants (though this one is about to undergo significant reshoots) -- and we imagine that Fox has/had sequel ideas in mind for all three films. Actually, we know that a New Mutants trilogy was planned, and yeah, Deadpool 3 was all but inevitable. But now that the characters of the X-verse are set to return to Marvel, pretty much every project outside of those first three is being called into question. And that's a lot of projects.

Related: All 13 X-Men Films Fox Still Has In Development

Despite constant delays and enough director turnover to make the Flash movie look like it's experiencing a smooth pre-production phase, the Gambit solo film starring Channing Tatum still has a release date (April 2019). Another long in-development project, X-Force, has been in the works for nearly 5 years now, but after finally locking down writer/director Drew Goddard last September, plans were in place to start filming as early as this year. These two still have a solid chance of happening, but things get far less concrete after that. Solo movies are currently in the works for the likes of Doctor Doom, Kitty Pryde, Multiple Man, and X-23. And those are just the projects we know about. Though the status of those films hasn't been officially confirmed one way or the other, as of now, they're iffy at best.

Prior to the Disney deal, Fox had the next 2-5 years of its X-verse (at least vaguely) mapped out. We're not saying that Marvel needs to wait on all that once the deal goes through; they definitely won't. But it may be in Marvel's best interest to work hand in hand with Fox on which projects to develop, which would help smooth the transition once the ink dries on the merger. That is, assuming that Marvel doesn't have too much on its plate as it is.

The MCU also has A LOT going on right now

Marvel has spent nearly a decade building their cinematic universe, and its all set to culminate this summer with Avengers: Infinity War. After the third Avengers movie and Black Panther hit theaters, Marvel will follow up with Ant-Man and the Wasp, Captain Marvel, Avengers 4, Guardians of the Galaxy Vol. 3, and a sequel to Spider-Man: Homecoming. For everyone keeping score at home, those last two will kick off Phase 4 of the MCU, and the full MCU schedule is concretely set through 2019. But hey, the phases usually last 2-4 years and consist of 6-10 films, right? There's plenty of room to introduce the X-Men and the Fantastic 4 somewhere in there, right?

Related: 5 Awesome Villains The MCU Can Now Use Thanks To The Fox Deal

The problem there is that it takes at least two to three years to develop a movie. Fox's Marvel characters won't actually be accessible until mid-2019, which means that we won't likely see them onscreen in anything more than a teaser/cameo role until 2021. If they wanted to do it earlier, the MCU would have to start developing projects centering on characters they don't even officially have the rights to yet, which is an awfully risky thing to do. There's really no sense in rushing this, assuming that Marvel has plans in place to keep their cinematic universe firing on all cylinders until the X-Men arrive.

Which brings us to the small matter of Marvel having 20 friggin movies on the docket before the Fox deal was agreed to. (When Kevin Feige confirmed that number in late November, he was seemingly including Spidey 2 and Guardians 3.) Since the statement was made just weeks before the Fox deal was confirmed, it's possible that he may have had an X-related project or two in mind as well. But again, that's not definite, and the MCU has been making some serious moves as of late to expand its universe and introduce new heroes. Marvel's dance card will never be too full for the X-Men, but it's hard to see there being much room to squeeze them in before Phase 5 without them completely reworking all the plans they have in place.

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Simply put, Marvel won't likely have the rights to the X-Men and the Fantastic Four officially secured before Phase 3 ends with Avengers 4. And since both Fox and Marvel have so much on their plates as it is, we shouldn't expect to see the two teams join the MCU before Phase 5. If they arrive sooner, it could end up being a rush job. We've waited over a decade for this -- we can wait a few more years to ensure that it's done right.

Next: Marvel & Fox Are Using Plot Devices That Can Merge Franchises Together

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