One idea could be that Ben Tennyson's Omnitrix malfunctions while he's in the Avatar world. So, instead of transforming into his usual aliens, he starts taking on the forms of Avatar creatures. For example, he might turn into a flying bison or a waterbender - like spirit. This would create all kinds of chaos and new adventures as he tries to figure out how to control these new forms and also deal with the political and elemental challenges of the Avatar world.
Maybe in the fanfiction, the Avatar and Ben team up to fight a common enemy. The enemy could be an alien race that has somehow found its way into the Avatar universe and is causing havoc by disrupting the balance of the elements. Ben could use his various alien forms to complement the Avatar's bending abilities. For instance, Heatblast could help deal with fire - based threats, while the Avatar uses waterbending to counter other attacks.
One idea could be a crossover between Avatar: The Last Airbender and Harry Potter. Imagine Aang and his friends attending Hogwarts and learning magic alongside wizardry students. They could use their bending skills in magical duels.
Another idea is to have Ben 10 cross over with a futuristic sci - fi setting. His harem could be made up of female alien species he encounters. They could be intrigued by his ability to transform and his heroics, leading to a harem - like situation where they all vie for his attention.
Well, a cool concept for a Ben 10 Transformers crossover fanfiction could be that the Transformers' AllSpark has a strange effect on the Omnitrix. This makes Ben able to transform into new alien - Transformer hybrids. For instance, Four Arms could become a huge hulking form with robotic arms that can transform into different weapons. Also, the Decepticons might try to steal the Omnitrix to use its power to create more powerful soldiers. Meanwhile, the Autobots would team up with Ben to stop them. Maybe there could be a situation where Ben has to use his alien forms' unique abilities in combination with the Autobots' skills to defeat a powerful Decepticon threat.
One idea could be that Albedo creates a device that can swap powers with Ben, leading to a lot of chaos as they both struggle to adapt to the new powers. Another could be that Albedo and Ben are forced to team up against a new powerful enemy that threatens both their worlds. A third idea might be that Albedo gets stuck in Ben's body somehow and has to pretend to be Ben while dealing with his friends and family.
Perhaps Ben teams up with the JLU to fight a common enemy that is immune to the Justice League's normal attacks but can be affected by some of Ben's alien forms. They could start off not trusting each other, but as the story progresses, they learn to work together and combine their strengths. Ben could use his alien forms like Four Arms to provide brute strength in battles, while the JLU members use their strategic skills.
Perhaps the Akatsuki from Naruto somehow get involved in the Avatar world's politics. Their goals of collecting tailed beasts could be translated into a search for some powerful Avatar - related artifacts. This would stir up a lot of chaos in the Avatar world and force the Avatar and his friends to team up with some Naruto characters to stop them.
One idea could be that Gaara, with his sand - controlling abilities, arrives in the Avatar world during a time of great turmoil. He could team up with Aang and use his sand to help in battles against the Fire Nation. Maybe his sand could also be used in creative ways to counter the bending of the various elements in the Avatar universe.
One idea could be that Ben uses his Omnitrix to transform into alien forms that have unique abilities to survive in the Star Wars universe. For example, a heat - resistant alien to withstand the heat of Tatooine's deserts or a species with enhanced senses to navigate the dangerous jungles of Kashyyyk. Another idea is that the Galactic Empire somehow gets hold of the Omnitrix and tries to use it for their own evil purposes, leading to Ben having to team up with the Rebel Alliance to get it back.
Maybe the Omnitrix gets malfunctioning and starts to attract Grimm. The RWBY team has to protect Ben while he tries to fix it. During this time, they could learn about each other's powers and cultures. Ruby might be fascinated by the different alien forms and how they can be used in combat, and Ben could be impressed by the Dust - based technology of RWBY's world.
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.