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 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.
Another interesting pairing could be Ben and Captain America. Cap's leadership and moral compass could influence Ben, while Ben's diverse powers could be a great asset to Cap in battle. They could be in a situation where they have to fight against a mind - controlled army, and Ben's aliens like Big Chill could freeze the enemies in place while Cap gives strategic commands.
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.
I think in 'Marvel The Flash Crossover Fanfiction', the most interesting part would be how the Flash's abilities are perceived in the Marvel universe. Since the power scales and the overall feel of the two universes are different. Maybe the Flash would be seen as an anomaly, or perhaps a new kind of hero that the Marvel characters have to adapt to. It could also explore how the Flash's speed affects time and space in a world that already has characters like Doctor Strange who are well - versed in such concepts.
Well, it could be about a crossover in the Ben 10 universe that involves neglected or overlooked characters or storylines. Maybe it's a fan's attempt to give more spotlight to those parts that the official shows or comics didn't fully explore.
One idea could be that Naruto is transported to the Marvel universe and his godlike powers are mistaken for being a new type of mutant. The Avengers might initially be wary of him, but as they see his sense of justice and his willingness to help, they start to work with him. For example, his ability to create countless clones could be used in battle strategies against villains like Thanos.
Once upon a time in a world where Autobots and Avengers co - existed. Optimus Prime teamed up with Iron Man. They were facing a new threat that combined the technology - stealing abilities of the Decepticons and the magic - misusing powers of some Marvel villains. Together, they used their combined intelligence and strength to fight back. Optimus's strength and Iron Man's high - tech gadgets made them a formidable duo.
A cool scenario would be a battle royale - style crossover where Wonder Woman is brought into the Marvel universe to participate in a tournament. She has to fight against various Marvel heroes and villains, like Thor and Loki. As she progresses, she discovers that the real mastermind behind the tournament is trying to use the chaos to open a portal to another dimension. Along the way, she forms alliances with some of the Marvel characters and together they try to stop the evil plan.