Out of the Fox properties that Marvel Studios has gained the rights back to, they're starting with Fantastic 4 before bringing the X-Men into the MCU.
Marvel Studios is prioritizing the Fantastic Four over X-Men when it comes to who is getting their MCU debut first out of the Fox properties. Ever since Walt Disney Studios acquired 20th Century Fox, there have been a lot of speculations on which of those characters would join the MCU first. With most of Phase 4 already filled out, there were theories that perhaps the X-Men and Fantastic Four would not join the larger universe until Phase 5. At San Diego Comic-Con 2019, Marvel Studios president Kevin Feige even teased that there were plans for those properties but that they "ran out of time" at the big Hall H presentation.
Given the current global pandemic, there have been a few shake-ups for Marvel's slate for Phase 4 as projects have been changing release dates. That also applies to the several Marvel Studios shows that are coming to Disney+. However, Disney just had their big Investors Day where a lot of surprises were unveiled for the future of Marvel Studios. One big announcement that many didn't see coming was Feige announcing that Spider-Man director Jon Watts will officially direct the Fantastic Four for Marvel Studios. While no release date, cast, or writer was announced, the iconic team is on their way to the MCU.
Unfortunately, for all the eager X-Men fans across the world, nothing was said about when the MCU will introduce the Marvel's mutants. While Marvel and Disney have yet to specify any official reason for why they haven't been brought in yet, there are a few key factors to take into account and one of them is no doubt proximity. Before the acquisition, X-Men: Dark Phoenix was the final installment in Fox's X-Men Cinematic Universe to be released in theaters. From the enormous box office disappointment as well as the film's negative reception among critics, X-Men: Dark Phoenix was a disastrous finale for Fox's X-Men franchise that started in 2000. Releasing too close to that release would be a dangerous prospect.
While The New Mutants did get its poor-received release, it may not quite fit with the legacy of the original X-Men films, but it did include mutants and it is too soon to bring them in the MCU on the back of that. Given that the MCU has barely even acknowledged the Inhumans that were introduced through Agents of S.H.I.E.LD., that group is a priority for Marvel Studios than the X-Men since Ms. Marvel is coming to Disney+. In contrast, the Fantastic Four have had less big-screen presence as they only had a two-film series as well as a reboot that bombed critically and financially. As with the Inhumans, the Fantastic Four also have priority behind them because they hold the title of being Marvel's First Family and are an iconic part of the comics history. They aren't being brought to the MCU in place of the X-Men to serve only mutantkind, they deserve high billing on their own.
The last time the Fantastic Four appeared on the big screen was in 2015 so by the time they return to the MCU, there will have been at least 6 years of distance from Josh Trank's Fantastic Four. Add to that factor, the very real selling point that they can bring something entirely new into the MCU that other films haven't focused on too much. That's not to say that the X-Men project doesn't matter, but it would be foolish not to acknowledge the difficulty posed by the proximity to the just-ended Fox franchise. And while little is known about Marvel Studios' Fantastic Four film, it'll be an interesting couple of years to follow as Disney begin putting the project together that will hopefully come with some news for the X-Men too.