Spoiler alert: This article contains spoilers for Doctor Strange in the Multiverse of Madness
The X-Men have begun to make their way into the MCU, as seen in Doctor Strange in the Multiverse of Madness, so the doors are open for Marvel's mutant characters who never had the chance to make their live-action debuts in Fox's X-Men film series. With the likes of Wolverine and Deadpool dominating the Fox franchise, not every character from the deep X-Men bench got a chance to shine, even many of those who did appear in the movies, albeit briefly.
Still, a little bit of screen time is better than none at all, and there are plenty of X-Men heroes and villains who never got the chance to appear on the big screen. Now that the MCU is in a position to right those wrongs, there are certain iconic characters that X-Men fans should expect and even demand to see next.
Mister Sinister
Mister Sinister is a longtime foe of the X-Men and an evil scientist who experiments on humans and mutants. The mutant geneticist has some command over his own genetics, granting himself virtual immortality and enhanced durability.
Comic book fans know how well Mr. Sinister lives up to his name and how far he will go to torment Cyclops and Jean Grey. He was teased to make his debut in the Gambit movie, but that film is no longer in the works, with the rights going to Marvel Studios.
Daken
The son of Wolverine, Daken has the same healing and retractable claws as his father. Separated at birth, Daken was told that Wolverine was the one who killed his mother, and he was raised as an agent and assassin before turning good and taking up the mantle of Wolverine himself.
The only child (of sorts) that Wolverine had within the Fox franchise was X-23. Despite 2013's The Wolverine taking the character to Japan, there was no lead to him meeting and having a child with his wife Itsu.
Onslaught
Onslaught is the combination of Professor X and Magneto as he is made through the consciousness of both. With Magento's magnetism powers and Professor X's mental powers, Onslaught looked to create Professor X's world of peace through Magneto's methods of violence.
Onslaught has Omega-level powers that make him a threat to the X-Men and the world. Moviegoers got to see Xavior and Magneto work together but never with this level of power.
Northstar
Foxed missed out on using a character that might be coming to the MCU in the member of Alpha Flight, Northstar. Although not the leader of Alpha Flight, what makes Northstar a great character is an impact he had on the LGBTQ+ community by being the first Marvel character to come out and receive a same-sex wedding.
In a team hailing from Canada, Northstar has the power of flight and super speed. Being a mutant himself, he along with Alpha Flight have a friendship with the X-Men, serving as allies at times.
Proteus
Kevin MacTaggert is a mutant with omega-level powers that even he can't contain. Proteus lived in a Mutant Research Facility under energy to keep prevent psionic energy power from consuming him and finding a new host body.
Freed from his cage by Magneto, Proteus is a villain that grows stronger as he eventually gains the power to warp reality. Being experimented on and having a body that harms himself could have made him a sympathetic and dangerous villain in the X-Men movies.
Firestar
Making her debut in Spider-Man and His Amazing Friends alongside one of the first members of X-Factor, Iceman, Firestar is a mutant who uses microwave radiating radiation to fight crime. Her power of radiation allows her to control heat.
Firestorm is a proven team player with years working with Spider-Man as well as a sometimes member of the Avengers and the X-Men as well as a founding member of the New Warriors. With so much team history in the pages of Marvel Comics, there's lots to draw on for Firestar to serve as a bridge between established characters with new ones in the MCU.
The Maker
The Maker's true identity of Reed Richards is in the Fox franchise but only as his hero persona of Mr. Fantastic. The audience never got to see the villainous side of Reed Richards that has been shown in the comics.
From Earth-1610, Reed Richards lets his ego get to him as he wishes to in a sense, conquer the other Earths. Rejected by Susan Storm and guilty for the death of Dr. Doom, Reed Richards went to another universe as The Thinker to gain a sense of control of his life through conquering others.
Cynthea Von Doom
Fox made changes to Dr. Doom's character by relying on the scientific aspect of him and making him a power-hungry egotistical villain. In the comics, he was from Latveria and was motivated by freeing his mother Cynthea Von Doom from hell.
Cynthea was a Gypsie sorceress who made a deal with Mephisto to overthrow the Latveria ruler, bounding her to hell. The death motivated Doom to learn both science and magic to rule Latveria and free his mother.
Annihilus
An insect-like being, Annihilus is a conqueror and destroyer that comes from the Negative Zone. His rule of the Negative Zone is represented by the Cosmic Control Rod that rules over the Annihilation wave.
The Negative Zone has appeared in both versions of the Fantastic Four movies, but neither included the likes of Annihilus. The Negative Zone can appear in the MCU and this time Annihilus can appear as the leader.
Franklin and Valeria Richards
Franklin and Valeria Richards make up Marvel's first family as they are the children of the Fantastic Four's Mr. Fantastic and Invisible Woman. Their oldest son is one of the most powerful mutants on Earth as he has the power to perceive the future, reshape reality, and many more. Franklin would use his reality-warping powers to save his little sister Valeria from being stillborn. She would be raised in an alternate future where Gains the power of force field from her mother and her superhuman intelligence from her now father Dr. Strange.
Fantastic Four: Rise of the Silver Surfer had Reed and Sue get married at the start and ended the movie with the announcement that she was pregnant. However, a third movie was never made which prevented the arrival of Franklin and Valeria Richards. Franklin being a powerful omega-level mutant and Valeria the daughter of Doom doesn't seem like something that would have happened in Fox's Fantastic Four universe but may have a chance of happening in the MCU.