Ant-Man and the Wasp: Quantumania originally had more of Jonathan Majors, but Kang's backstory was heavily reduced.
The future of Jonathan Majors with Marvel is very much up in the air at the moment, but it turns out that the studio has been reducing his role for longer than thought. According to writer Jeff Loveness (via Backstory Magazine), the role of Kang the Conqueror was heavily reduced in Ant-Man and the Wasp: Quantumania, largely to remove backstory. Loveness specifically said that:
There was a lot more Kang backstory, a lot more Kang lore that didn’t get in there that we thought was maybe distracting or better served in an Avengers movie.
While Jonathan Majors as Kang the Conqueror has been heavily promoted as the new Big Bad of the Marvel Cinematic Universe, it seems the studio is willing to take it a bit slow. It makes sense that the MCU would want to build up to the concept of a Multiversal threat, especially when Kang himself is a pretty difficult problem for the cohesion of the overall narrative. But it is interesting that the very first movie in which Jonathan Majors appeared in a Marvel role would already be cutting him down.
It seems that Jonathan Majors’ Marvel role was reduced in Ant-Man 3 in favor of the same lore being introduced in a future Avengers movie. Jeff Loveness has been announced to write Avengers: The Kang Dynasty, so it can be assumed the amount of Kang origin stories cut from Ant-Man 3 will show up in some form in that film. Of course, considering there is an infinite number of Kangs to give backstory to, that might take a while.
That also seems to be part of the decision to leave a whole bunch of Jonathan Majors on the cutting room floor with his Marvel debut. Also according to Loveness, there were concerns that delving deeply into the backstory of the notoriously convoluted villain would slow down the pace of the movie. Given that Ant-Man 3 was tepidly received by both critics and audiences (not to mention a historically bad box office drop in the second weekend), that was probably a good decision.
There is no word yet whether Jonathan Majors will continue working with Marvel Studios after the actor was recently arrested for allegedly assaulting an individual described as a domestic partner. The actor’s legal team has claimed to have evidence exonerating him from the allegations (including apparent texts from the alleged victim), but it is unclear if he will retain his very prominent role in the coming Phases. If Marvel does decide to replace the actor playing Kang, it could be a whole lot more difficult for them than a character known for having many different variants.
Jonathan Majors is currently slated to return to Marvel in the second season of Loki, the Disney+ series starring Tom Hiddleston as the God of Mischief and Owen Wilson as Mobius M. Mobius, agent of the Time Variance Authority. We shall just have to see how much of Majors gets cut out this time.