With What If...? returning for season 2 in 2023, Marvel certainly does have some solid modern animation credits. However, DC has largely won the comics war when it comes to cartoons.
The one Marvel series that has pumped out multiple high-quality cartoons in the last two decades is X-Men, with Wolverine and the X-Men and X-Men: Evolutions both being fantastic works. If the two were combined, and fitting DC cartoon characters with mutant-like powers got to be X-Men, there would be no better cartoon out there.
Plastic Man
Despite not being one of the characters most heavily associated with the JLA, Plastic Man is one of the Justice League's best characters whenever he's given the spotlight he deserves.
While the X-Men in cartoons are usually portrayed as in their mid to late teens, there are also often characters like Storm and Beast that are fully fledged adult mutants. Plastic Man would be a welcome addition to this group of characters, and his interactions with mutants like Wolverine would be hilarious to see.
Matter-Eater Lad
Matter-Eater Lad was one of the stranger minor characters in Legion of Superheroes, with the surprisingly overpowered ability to eat absolutely anything and everything.
While he was mostly used as a joke character in the cartoons, Matter-Eater Lad has had some incredible moments in comics. His age and bizarre powers would make him a perfect fit in the X-Men as a minor character. In particular, it'd be hilarious to see Matter-Eater Lad's assistance eating entire metal hunks in a fight with Magneto.
Jinx
Jinx was both a villain and a crucial member of the Teen Titans, one of animation's best supergroups, throughout the acclaimed series. In particular, her relationship with Kid Flash was one of the better minor storylines.
Mutant powers like Jinx's aren't unheard of in X-Men, and she'd have been a fantastic replacement for Scarlet Witch. Wanda is pretty heavily ingrained in the Avengers and MCU, meaning a new sorceress type would be a perfect addition to replace her role within the X-Men.
Aqualad
One of the select few to be the leader of the "The Team" in Young Justice, Aqualad was a fearless leader. Similar to Robin from Teen Titans, Aqualad even pretended to change sides for a time to help save his friends.
While Aquaman and Namor types have been ridiculed and parodied in plenty of superhero media, Aqualad's hydromancy abilities feel like a very grounded mutant power that would fit in well with the X-Men. His fantastic dynamic with Nightwing in Young Justice would also translate well to the type of friendship he could have with Cyclops.
Kid Flash
Kid Flash has made an appearance in plenty of superhero media, but the sidekick of Flash is best known for his charismatic portrayals in Young Justice and Teen Titans.
The quippy speedster would admittedly have some diminishing returns in the X-Men universe, courtesy of Quicksilver, but two speedsters on opposite sides are nothing new in comics. Should Quicksilver, as he often has been in X-Men animation, be portrayed for his more villainous side, Kid Flash would have made a great foil.
Timber Wolf
Legion of Superheroes brought plenty of fantastic young heroes, but one hero, in particular, would have an especially interesting place in the X-Men world: Timber Wolf.
Timber Wolf is actually a mutant in Legion of Superheroes, although a mutant by scientific experimentation rather than genetics. Were the powerful wolfman in the X-Men, he'd have a fascinating relationship with one character in particular: Beast. Henry McCoy could be a fantastic mentor and father figure to the troubled Timber Wolf, and they'd have much to empathize with.
Terra
Terra is both the most complicated and underrated Teen Titans characters in the original series. Her relationship to both Beast Boy and Slade made her fascinating to watch.
In the X-Men, Terra would be a fantastic mutant with earth powers. She'd be a far superior inclusion to an X-Men cartoon than Avalanche from X-Men: Evolutions, who served as a fairly mediocre field leader to the Brotherhood of Mutants and odd love interest to Kitty Pride. Terra, however, would be a strong and powerful inclusion to the franchise.
Static
The protagonist of Static Shock and a minor hero in Young Justice, Static is one of the coolest superheroes with electric powers. While most young superhero shows focused on groups or already big names, Static carried his show by himself on charisma and depth.
Unlike many DC heroes who often take on planetary threats, Static often operates on a very local level. Along with his best friend and crime-fighting partner Gear, Static felt more on an X-Men level scale in his show. Personality-wise, both Static and Gear would add a lot of fun and casual energy to an X-Men cartoon.
Brainiac 5
One of the main characters of Legion of Superheroes, Brainiac 5 is the descendent of one of the JLA's greatest enemies: Brainiac. Brainiac 5 is also one of the greatest cartoon robots out there.
It's a shame that no X-Men cartoon has a solid robot character, given the incredible technology in the world. DC is already full of fantastic robots and cyborgs, but most of the great robotic characters in the Marvel universe are villains. Brainiac 5 would offer the X-Men an interesting n0n-mutant and something Marvel as a whole is lacking: a great robot.
Beast Boy
Arguably the most beloved character in the original Teen Titans cartoon, Beast Boy was a former Doom Patrol member who became the lovable heart of the Teen Titans.
Beast Boy's powers would fit perfectly in the X-Men world. Not only would he be a unique character personality-wise and power-wise, but he would have all sorts of great possible interactions right away. He and Nightcrawler could relate to their appearances, and he'd be a handful for Cyclops and Wolverine to try to keep in line.