There are many ways in which this has happened already, Doctor Strange in the Multiverse of Madness saw Patrick Stewart reprise his role as Professor X from the Fox films, while Hugh Jackman is set to return as Wolverine in Deadpool 3.
So how will the characters help redefine the MCU? Well let's dive into it.
X-Men: How the iconic superheroes will change the MCU
Marvel has been planning to bring in the X-Men into the MCU for some time, with the possibility of their return becoming a possibility when the Walt Disney Company acquired 20th Century Fox in 2017.
Read more: Should Marvel revive the original Avengers?
Ever since there have been small rumblings of the mutants in the MCU, the first direct reference appeared in 2022's Ms Marvel when Kamala Khan (Iman Vellani) was described as having a mutant gene. Black Panther: Wakanda Forever revealed that villain Namor (Tenoch Huerta) was also a mutant later that year.
Elsewhere, the MCU's most recent release The Marvels revealed the first appearance of another member of the X-Men: Dr Hank McCoy. The character is played once again by Kelsey Grammer, who portrayed Beast in the original Fox movies — Nicholas Hoult took on the role in the reboot.
WandaVision also featured a Red Herring in the appearance of Evan Peters, who said he was Wanda Maximoff's brother Peter but who was later revealed to be her neighbour. The actor portrayed the character Quicksilver in the prequel X-Men films, hence why his role in the Disney+ series caused such excitement amongst fans.
With the X-Men slowly being brought into the MCU it begs the question what will it mean for the franchise?
Beloved characters add nostalgia
The X-Men have a long track record on the silver screen, three films were released in the original film series from 2000 to 2006, and four films in the prequel series from 2011 to 2019.
Jackman's Wolverine had his own standalone trilogy, and there was a New Mutants spin-off as well as two Deadpool films before Fox was acquired by Disney.
The characters are beloved by moviegoers, which has helped cement their endurance in the movie industry and led to them being brought back time and again.
Earlier this year there were reports that Marvel Studios were looking for ways to renew interest in the MCU, with this being linked to a possibility of the original Avengers returning to the fold, but really the X-Men would add a lot of the nostalgia that is needed to do this.
Many Marvel viewers are already avid fans of the X-Men, and people are waiting to see them join the MCU proper so this would no doubt bring back many moviegoers to the cinema.
A boost to the box office?
The MCU has struggled at the box office in recent years despite previously dominating the industry, with superhero fatigue and a lack of big names like Iron Man and Captain America in the MCU leading to a general downturn in ticket sales.
With the introduction of the X-Men, Marvel bosses are no doubt hoping the combination of familiar characters and nostalgia for the original Fox movies will generate more interest in new films, giving the MCU box office the boost it so desperately needs.
The original X-Men film was a huge commercial success, earning $296.3m worldwide, and it helped kickstart a re-emergence of the superhero genre — so what's to say the characters can't do that again for the MCU?
The superheroes the MCU could use
Since the release of Avengers: Endgame, several big hitters in the franchise have departed including Robert Downey Jr., Chris Evans and Scarlett Johansson.
Since then, Marvel has been introducing new characters and setting the stage for existing characters to take up the mantle of their predecessors. Ultimately introducing a lot more heroes to viewers and expanding the MCU bigger than it ever has been before as it hurtles towards Avengers: The Kang Dynasty and Avengers: Secret Wars in 2026 and 2027.
On paper this works well, but in reality the speed at which the MCU has expanded has led to a general feeling of disconnect for many viewers who either can't keep up or don't want to.
The X-Men are heroes who could really come as a help to those left in the MCU, within it their powers and pre-existing team dynamic will no doubt be of use when Kang the Conqueror (Jonathan Majors) sets out on his big plans for the takeover of the multiverse. Outside of it, viewers will likely be drawn to characters they already know.
The MCU is going to need all the help it can get in order to cross the finish line, and the X-Men are the heroes that are capable of doing it.
The Marvels is out in cinemas now.