One idea could be Ben Tennyson using his Omnitrix to team up with Spider - Man. Ben could transform into XLR8 to chase down villains while Spider - Man swings through the city. Another could be having Four Arms help the Hulk in a battle, combining their brute strength. And perhaps Heatblast could join forces with the Human Torch to fight a fire - based villain.
Well, a cool crossover might be Ben's Ghostfreak form working with Doctor Strange. They could both deal with other - dimensional threats. Maybe Ben as Diamondhead could assist Iron Man in building some super - strong structures. Also, Ripjaws could team up with Namor to explore the underwater parts of the Marvel universe.
One idea could be that Ben Tennyson's Omnitrix gets mixed up with some of the Infinity Stones' energy. This causes his aliens to gain new powers related to the Stones. For example, Four Arms could have super strength on a whole new level like Hulk. Another plot could be that the heroes from both universes have to team up to fight a villain who is using technology from the Ben 10 world and magic from the Marvel universe. Maybe a new character who has stolen some of Doctor Doom's knowledge and some of Vilgax's power.
One popular idea is crossovers with other popular animated shows like Teen Titans. It's exciting to see the Ben 10 characters interact with the Titans. Another idea could be a crossover with the Powerpuff Girls, imagining how the different powers and personalities would clash or cooperate. Also, crossovers with the Justice League can be really cool, as Ben 10's alien forms could bring a new dynamic to the superhero team.
Another interesting plot could be that some of Ben's alien forms are mistaken for new mutants in the Marvel Universe. He then has to prove that he's not a threat while also helping the X - Men deal with a common enemy that is interested in the genetic makeup of both mutants and Ben's alien forms.
A really interesting 'ben 10 marvel crossover fanfic' could be the one where Ben gets transported to the Marvel Universe during the events of Civil War. He has to decide which side to take, if any. His neutral stance is challenged as he realizes the implications of the Superhuman Registration Act on his own powers and identity.
Team - up and friendship is also a major theme. Ben, coming from his own universe, has to form alliances with Marvel characters. These relationships develop as they face various threats together. For example, Ben might become friends with Iron Man and learn about technology and responsibility from him.
One idea could be a story where the Avengers and the Justice League team up to fight a common enemy like Darkseid and Thanos working together. Another could be a character - swap scenario, for example, Superman being in the Marvel universe and having to deal with the likes of Doctor Doom. Maybe a story where Batman uses his detective skills to solve a mystery in the Marvel world that involves Hydra.
One great idea could be a crossover between the X - Men and the Avengers. Imagine Wolverine teaming up with Captain America, their different fighting styles and moral compasses would create an interesting dynamic. Wolverine's rough - and - tumble, lone - wolf approach contrasting with Captain America's more traditional leadership and sense of justice.
One idea could be to have Ben Tennyson end up in the Young Justice universe by accident. Maybe his Omnitrix malfunctions during a fight and transports him there. He could then team up with the Young Justice heroes to fight a common enemy like the Light or some new villain that's a threat to both universes.
One idea could be that Master Chief gets transported to the Marvel universe. He teams up with the Avengers to fight a common enemy like Thanos who has somehow obtained Covenant technology. His Spartan abilities would be a great addition to the team.
Well, a cool concept would be for Luke Skywalker to end up in the Marvel universe and train some of the young superheroes in the ways of the Force. Or perhaps the X - Men could find themselves in a conflict in the Star Wars galaxy, with their mutant abilities adding a new dynamic to the Star Wars battles.