Reddit has some strong opinions on this topic. Some examples they give do have a chance to enter the MCU at some point, due to the mutants' relationship with already existing characters (Marrow interacting with Spider-Man). The majority of examples Redditors give are due to their powers, complex storylines, sensitive material, or similarity to already existing Marvel and DC characters.
Content Warning! The following article contains discussions of sexual assault.
Redditor JeanGreyStan143 kicks off their own discussion thread with an interesting suggestion. They propose that Madelyne Pryor would be difficult to introduce in an MCU movie, due to her storyline being too convoluted. Despite saying that it might work as a TV show, CrawdadMcCray disagrees, retorting that she can't be introduced until Jean Grey is dead.
Both Redditors doubt her appearance as she is a clone of Jean Grey created by Mr. Sinister. Her inclusion seems unlikely as she is already too similar to Jean, and while new exciting characters could be introduced (Mr. Sinister and Cable), her storyline is too complex and melodramatic to fit with the rest of the X-Men.
It can be argued that there are far too many mutants already, which is why Redditor mrsunrider wants a moment of silence for Soft Serve. This Redditor is clearly making a joke, but nothing compares to the humor that Soft Serve "provides."
Originally starting as a fan fiction mutant, Soft Serve is now canon (via Screen Rant). Her power is to poop ice cream. While this raises many questions as to how that power actually works (audiences may not want to know), just thinking of Soft Serve fighting alongside the X-Men is somewhat disgusting yet pretty comical. Even though Marvel has leaned more into the realm of comedy with movies like Thor: Love and Thunder, her appearance in the MCU smells rotten.
There are some mutants who shouldn't be introduced into the MCU mainly due to their powers, and Passerby05 feels that ForgetMeNot is one of those mutants.
ForgetMeNot, who was one of Marvel's most powerful X-Men members once upon a time, has the perfect name considering what his power is. Once he disappears from line of sight, he is completely forgotten, with those around him having no memory of his existence or what they were currently doing. Naturally, this would cause problems for not only his enemies but for his allies as well. His inclusion would hinder the writing as he serves as a band-aid for plot holes. ForgetMeNot is silly even by comic book standards.
There are some mutants with unique powers and abilities, and then there are some mutants with less impressive powers and abilities. It could be the latter as to why Redditor XMenChangedMyLife feels that Catseye "will probably never show up on the big screen."
Catsye's power is ailuranthropy which gives her the ability to become a cat. She can turn into a cat or a cat-human hybrid. This is similar to Dracula, where he can turn into a bat or a combination of man and bat to become a vampire. Perhaps the biggest reason why Catseye won't be among her fellow mutants is that she is too similar to her DC counterpart in Cheetah. They have similar abilities and looks, which could lead fans into making non-stop comparisons.
There are many intriguing mutant villains in Marvel, yet Aspiring_Sophrosyne believes that The Mandrill in particular will not make an appearance in the MCU.
The reason he will not make an appearance is due to his powers. Mandrill has the ability to release pheromones that give him the ability to entice and enslave women. This misogynistic idea (which didn't make much sense) was already presented in Black Widow with the main villain, Dreykov. Like Dreykov, Mandrill has used this ability to control women as an armed force. However, Mandrill has taken advantage of these enslaved women, even marrying some of them. For these obvious reasons, Mandrill won't be an MCU villain.
The MCU has done a better job of introducing more diverse women into its films recently. However, ThanatosTheory believes that Stacy X will never be brought in. Fellow Redditor My_DnD_Account agrees, adding "This is probably the least likely character to be adapted."
Both Redditors are in agreement for one simple reason - Stacey X is a sex worker. She has the ability to control an individual's pheromones through skin contact. This creates sensations and physical reactions to those that she touches. Despite her also using this ability to heal others and infuse them with adrenaline during a fight, Stacey X most likely won't make an appearance in the family-friendly MCU.
Redditor drmikey88 gives a few examples of mutants least likely to enter the MCU, but one that stands out is Shark-Girl. Shark-Girl is a hybrid shark that can survive on land or sea. Much like an actual shark, when Shark-Girl smells blood, it turns into a feeding frenzy.
Introducing her would require a significant amount of blood and gore, something the MCU isn't known for nor is comfortable with. Much like Catseye, Shark-Girl bares a striking resemblance to King Shark in The Suicide Squad. Fans will be comparing the two, with King Shark likely being the victor, due to his popularity and being on screen first.
After the inception of the mutant nation of Krakoa, Charles Xavier and Moira MacTaggert formed a group of mutants in the hopes of resurrecting mutants who died prematurely. Known as "The Five," they consisted of Tempus, Proteus, Hope Summers, Elixir, and Egg.
Xygnux and TheBigDuo1 doubt the MCU would focus on this group of mutants, because of their lack of popularity. The release of X-Men '92: House of XCII complicates things as well, according to Screen Rant. Audiences are captivated by nostalgia, but as the Redditors point out, the introduction of The Five would only make "hardcore fans happy." The MCU made Guardians of the Galaxy popular, but they may not be able to capture lightning in a bottle twice.
A deleted Reddit user chose to stick to "bigger names" that the MCU could leave out, among them is Apocalypse. Known as being the first mutant, Apocalypse was the central villain in Fox's prequel X-Men: Apocalypse.
The Redditor suggests that Apocalypse might be "too similar to Thanos," even if they have different motivations and methods. There is validity to their claim, as Apocalypse is to the X-Men what Thanos was to the Avengers. It will be difficult now to have a villain of Thanos' stature. If Marvel does decide to eventually include him, it likely won't be for a long time, as fans might still have a bad taste in their mouths from the previous attempt.
Of the many mutants that Redditor iamthedave3 suggests won't appear in the MCU, chief among them is Maggot. Maggot is much like Popeye if Popeye used slugs to gain his strength rather than spinach.
Maggot's digestive system consists of two slugs, Eany and Meany, that burrow out of his torso and use powerful enzymes to quickly absorb solid objects. They transmit this energy back to Maggot by re-entering through his belly, thus giving him strength. Although his outward appearance looks cool, the imagery of this process is nightmare fuel and likely a big reason why Maggot will never be in the MCU.