In some fanfictions, the technology in Ben 10's world might be combined with Stark's technology in the Avengers. Maybe Ben's Omnitrix could be enhanced by Stark's knowledge, creating a more powerful device that helps them in a battle against an alien invasion. This would involve a lot of scientific discussions and teamwork between Ben and the Avengers, especially Tony Stark.
In some fanfictions, there might be a plot where the Avengers mistake Ben for a threat at first. But then they realize his good intentions when he helps them fight a common enemy. This adds an element of misunderstanding and then friendship, which is quite common and engaging in crossovers.
One interesting crossover could be Ben using his alien forms to help the Avengers in combat. For example, Four Arms could provide extra strength in a fight against a powerful enemy like Thanos. Another could be Heatblast teaming up with the Human Torch to create an even more intense heat-based attack.
One idea could be a crossover with the Powerpuff Girls. Ben could team up with the girls to fight a common enemy that is causing chaos in both their universes. Another could be a crossover with the Teen Titans. Ben's alien forms and the Titans' powers would make for some epic battles and cool team - up moments.
One interesting crossover could be Ben using his alien forms to help Batman fight Gotham's villains in a more unique way. For example, Heatblast could be used to deal with Mr. Freeze in a direct heat - based battle.
One interesting crossover could be a story where the Avengers and Autobots team up to fight a common enemy like Ultron who has somehow allied with the Decepticons. The Avengers' superpowers combined with the Autobots' advanced technology would make for an epic battle.
One interesting crossover could be the Flash from Arrowverse meeting Quicksilver from the Avengers. They could have a race to see who is truly the fastest. Another could be Green Arrow teaming up with Hawkeye for an archery - based mission where they share their different techniques and arrow - making skills.
One interesting crossover could be the Avengers teaming up with the Miraculous heroes to fight a common super - powerful villain. For example, Thanos could be a threat to both the Marvel Universe and the world of Miraculous. The Avengers' brute strength combined with the Miraculous holders' unique powers would make for an epic battle.
One interesting crossover could be Stiles using his wits to outsmart Loki. Stiles is known for his intelligence in his own universe, and facing off against Loki's trickery would make for an exciting story. Maybe he could use his knowledge of the supernatural to help the Avengers deal with magical threats.
One interesting crossover could be the Digimon characters helping the Avengers in a battle against a powerful common enemy. For example, Agumon could use his fire breath to assist Iron Man in taking down a large group of villains. Another could be where the Avengers visit the Digital World and learn about the unique rules and powers there. They might team up with the Digimon to protect the Digital World from a threat that could also spill over into the real world.
A possible crossover could involve the technology of Kingsman, such as their fancy suits and gadgets, being integrated into the Avengers' gear. The Kingsman's bulletproof suits and high - tech umbrellas could give the Avengers an edge in combat. Also, the different cultures and protocols of the two groups would create some interesting clashes and comical moments as they try to work together.