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.
Well, a cool crossover might involve Ben's Omnitrix getting the attention of Tony Stark. Stark could try to analyze it and perhaps even upgrade it in some ways. Meanwhile, the Avengers could learn from Ben's experience in dealing with various alien threats, which is quite different from their usual foes. Ben, on the other hand, could gain knowledge from the Avengers' teamwork and strategic planning, making him a more well - rounded hero.
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.
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 avengers and flash fanfic, there could be a team - up against a common enemy. For example, a villain who can manipulate time. The Flash with his speed and the Avengers with their diverse powers would have to work together. The Flash could be the first to engage, using his speed to scout and gather information. Then the Avengers could come in with their strategic plans. Hulk might be a bit confused by the Flash's quick movements at first but would soon realize he's on their side.
One interesting crossover could be the use of Ben's Omnitrix to transform into a species similar to a Star Wars alien. For example, he could transform into something like a Wookiee - like creature and interact with the Star Wars characters in unique ways. Another aspect could be the combination of the Force from Star Wars and Ben's alien powers. Maybe a character with the Force tries to steal the Omnitrix, leading to an epic battle.
Well, in a Ben 10 and Marvel Zombies fanfic, a great crossover would involve Ben's Omnitrix getting corrupted by the zombie virus. This could lead to some of his alien forms having zombie - like traits. Also, imagine Ben teaming up with the few remaining non - zombified Marvel heroes like Spider - Man. They could use Ben's shape - shifting abilities and Spider - Man's agility to outwit the zombies.
In some fanfics, the Hulk might befriend Fluttershy. It's an unexpected pairing but it works. Fluttershy's gentle nature could potentially calm the Hulk down when he's angry. And the Avengers would be introduced to the magic of Equestria, like the Elements of Harmony which could be a key factor in defeating a common enemy.
One interesting crossover could be the use of magic in the Avengers' world. For example, Harry Potter characters could teach the Avengers some basic spells to help them in their battles. Another aspect could be the characters' personalities interacting. Hermione might befriend Bruce Banner and they could exchange knowledge about science and magic.
A crossover might involve character pair - ups. Maybe Hawkeye could team up with Deadshot. They are both expert marksmen, and it would be cool to see how they compare their skills and different moral compasses while on a mission together. Another possibility is Black Widow and Harley Quinn. Their contrasting personalities, one being a highly - trained spy and the other a deranged but brilliant criminal, could lead to some really interesting interactions.
One interesting crossover could be the Avengers having to deal with the dinosaurs in Jurassic World. Maybe Iron Man uses his technology to try and control the dinosaurs or Thor uses his powers to fight off the really big ones like the T - Rex. It would be a really chaotic but exciting scenario.
The characters would likely start off a bit cautious. Ben might be in awe of the Avengers' fame and power. The Avengers would be curious about Ben's alien - changing abilities. They'd probably exchange stories about their battles at first.