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.
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.
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.
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.
A Ben 10 and Marvel fanfic can stand out if it plays with the concept of time and space. Since Ben has access to different alien forms which could potentially mess with the fabric of space - time, and Marvel has characters like Doctor Strange who are masters of time - related magic. You could have a story where Ben accidentally disrupts the space - time continuum in the Marvel universe and has to team up with the Avengers to fix it. This would not only create a unique plot but also allow for some really interesting interactions between the two universes' characters.
Definitely Loki. He's a fan favorite and can easily fit into various crossovers. His mischievous nature can add a lot of twists to a story. For example, if he were to cross paths with the Fantastic Four, he could play all sorts of mind games with Reed Richards while also being intrigued by Sue Storm's powers.
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.
In a Ben 10 and Naruto fanfic, their powers would be a great combination. Ben has the ability to transform into various alien forms, each with unique powers. Naruto has his chakra and his powerful jutsu. They could learn from each other. For example, Ben could teach Naruto about some of the alien technologies, while Naruto could teach Ben about chakra control and the ninja way.
Captain America would be a great addition. His leadership and moral compass could guide Ben in some tough situations. And then there's Wolverine. His healing factor and fierce fighting style would contrast well with Ben's more alien - based combat. They could have some epic battles side by side.