One possible plot could be that Ben 10's aliens end up in the Monster High world. For example, Four Arms could get into a friendly strength - testing competition with some of the stronger monsters at Monster High like the werewolves. It would be a clash of different kinds of powers and cultures. Ben, in his human form, might also have to figure out how to fit in and make friends with the unique students at Monster High.
A cool plot could be that some of the Monster High monsters end up in Ben's world. Draculaura, being a vampire, might be mistaken for an alien at first. Ben has to protect her from the authorities who want to study her. In the process, they discover that there are similarities between the powers of the aliens and the abilities of the monsters. Together, they start to explore how they can use these similarities to their advantage and protect both their worlds from potential threats.
A plot could involve the characters from High School DxD mistaking Ben for a new type of devil or supernatural being because of his alien forms. Ben has to navigate this new world while also trying to find a way back home. Meanwhile, the devils in High School DxD might be interested in the technology of the Omnitrix, leading to a conflict where Ben has to protect it from falling into the wrong hands.
Well, perhaps there's a science experiment gone wrong at Monster High that creates a rift in the multiverse, pulling Peter Parker into their world. He teams up with Frankie Stein, who is also interested in science. Together, they try to find a way to close the rift while also dealing with the social drama at Monster High. The vampires might be causing trouble because they don't trust an outsider like Peter, but in the end, he wins them over with his heroic deeds.
A cool plot might be that Voldemort somehow discovers the existence of Monster High and plans to use the unique powers of the monsters there for his own evil purposes. So, Dumbledore sends a group of Hogwarts students, including Harry, to Monster High to warn them and form an alliance. The Monster High students, led by Frankie Stein and Draculaura, join forces with the Hogwarts students. They then have to use a combination of magic and monster abilities to stop Voldemort's plan, which could involve some sort of dark spell that would not only affect the wizarding world but also Monster High if successful.
Another plot could be that an evil force in the RWBY universe discovers the existence of the Omnitrix from another dimension. They manage to create a device to pull Ben and the Omnitrix into their world. Ben has to adapt quickly to this new world while Team RWBY, who are initially suspicious of him, realizes that he could be an ally against this new threat. They join forces, with Ben using his alien forms in creative ways to combat the Grimm and the evil force's minions.
One possible plot could be that Ben, with his Omnitrix, accidentally lands in the Soul Society. He gets mistaken for an intruder at first. Then, he teams up with Ichigo to fight against a new common enemy that is threatening both the world of Bleach and the places Ben has to protect. His alien forms could provide unique abilities in battles against Hollows or other powerful foes in the Bleach universe.
One interesting plot could be that Ben, using his Omnitrix, accidentally lands in the Pokemon world. He teams up with Ash and Pikachu to stop an evil organization that is trying to use both Pokemon and alien technology. For example, they might be creating hybrid creatures using Pokemon DNA and alien powers. Ben's different alien forms could have unique interactions with the Pokemon. His Four Arms form could be really strong in Pokemon battles, while his XLR8 form could be used to quickly traverse the Pokemon world to reach different areas in time to stop the villains.
One idea could be that Ben discovers a strange Poké - Ball - like device during one of his alien adventures. When he activates it, it transports him to the Pokémon world. There, he teams up with Ash and Pikachu. His Omnitrix malfunctions due to the different energy in the Pokémon world, and he has to find a way to fix it while also helping Ash battle against Team Rocket, who are now interested in the power of the Omnitrix.
One possible plot could be that the Omnitrix somehow gets linked to the Green Lantern power source. Ben could end up with a new set of alien forms that are influenced by Green Lantern energy. For example, he might have an alien form that can create constructs like a Green Lantern but with its own unique alien twist.
Well, maybe the Omnitrix gets damaged and its energy somehow mixes with a Devil Fruit. This creates a new situation where Ben has to deal with having powers from both the Omnitrix and the Devil Fruit. And the Straw Hats are intrigued by this new power source and try to help Ben control it while also facing threats from other pirates who want this power for themselves.
A cool plot might be that a powerful dark wizard in the Fairy Tail world discovers a way to control some of Ben's aliens. Ben has to seek help from Fairy Tail mages to break the control and then they join forces. The Fairy Tail members are amazed by Ben's ability to transform and his alien powers, and they work together to stop the dark wizard's plan to take over both worlds.