Developing 'Ultimatum': Marvel may have found a way to introduce the X-Men and the Fantastic Four to MCU
Marvel Studios has had the rights to feature both the X-Men and the Fantastic Four in the Marvel Cinematic Universe for a while. Many fans speculated that the characters - long out of reach for Marvel due to licensing issues - would be teased in 'Avengers: Endgame', but no such thing happened. Many fans have been wondering when or how the characters will first appear in the MCU but they may now have their answer.
Marvel Studios' next big event is reportedly going to be based off of 'Ultimatum' - a comic where Magneto snaps after the death of his daughter, the Scarlet Witch, and attacks all of New York with a tidal wave. It is an event that brought together heroes from all corners of the Ultimate Marvel Universe, including the X-Men and the Fantastic Four, in order to stop Magneto before he could destroy the world.
The Ultimate Comics line was an alternate take on many Marvel superheroes, introducing familiar heroes in fresh, new settings. A lot of aspects of the Ultimate Universe were adapted into the MCU, such as Samuel L Jackson playing the traditionally white Nick Fury. Fury was responsible for the formation of the Avengers and the construction of the Triskelion - the Avengers' base of operations. 'Ultimatum' was a controversial event as many fans felt it was an over-the-top, violent story whose purpose was to kill as many heroes of the Ultimate Universe for little more than shock value.
However, the basic premise of the story - the heroes of the world uniting to stop the threat of Magneto - may be the perfect introduction for the X-men in the MCU. 'Ultimatum' was also revealed to have been orchestrated by Doctor Doom. That, combined with the Fantastic Four's suspicions that the tidal wave was created by Namor, also make the Fantastic Four an important part of the story.
It's unlikely that Marvel will be doing an entirely faithful adaptation of the comic, but it's an event that's appropriately big enough for the introduction of such important Marvel teams. Marvel is already in early development of the project, according to MCU Cosmic. While none of this has been confirmed, the website has reliably had the scoop on such projects as the 'Eternals' and the 'Hawkeye' series.
It should be noted that the villainous ULTIMATUM group that will be appearing in 'Falcon and the Winter Soldier' had nothing to do with the original comics event - but that might change as Phase 4 progresses. However, no official announcements have been made regarding this project yet and they might be a long way off.