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.
A plot could involve Ben's aliens being mistaken for new and extremely rare Pokémon. Some Pokémon trainers try to catch his alien forms. Ben has to figure out how to deal with this situation without revealing his true identity. Meanwhile, he meets a Pokémon that has the power to enhance the Omnitrix, and they embark on a journey together to unlock the new potential of the Omnitrix while also exploring the Pokémon regions.
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 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, perhaps there's a magical artifact in the Harry Potter world that is malfunctioning and causing strange rifts in the universe. Ben 10 stumbles upon one of these rifts and ends up in Hogwarts. He teams up with Harry Potter and his friends. Ben realizes that his alien forms have different reactions to magic. Heatblast, for instance, can withstand some of the dark magic spells better than his human form, and together they embark on a journey to fix the artifact and close the rifts.
One possible plot could be that Danny and Ben meet when a powerful interdimensional villain is causing chaos in both their universes. They initially misunderstand each other and get into a small fight. But then they realize they need to work together. Danny could use his ghost powers to phase through objects and surprise the enemy, while Ben could use his various alien forms to combat the villain's different attacks. For example, using Heatblast to counter a cold - based attack from the villain.
One possible plot could be that the Omnitrix somehow gets mixed up with the Power Rangers' technology. Ben, as one of the Power Rangers, has to figure out how to use both the alien powers from the Omnitrix and the Ranger powers to fight a new, super - powerful villain. For example, he could use Heatblast's powers in combination with the Red Ranger's sword to defeat an enemy that is immune to normal attacks.
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.
One possible plot could be that Ben Tennyson, while on a trip with his family, somehow gets transported to the world of Fairy Tail. His Omnitrix malfunctions due to the different magical energies in that world. The Fairy Tail guild members are initially suspicious of him but when they see his alien forms in action against some common enemies like the Dark Guilds, they start to work together. Ben could learn about magic from the guild members and they could learn about his advanced technology.
Well, one possible plot could be that Ben Tennyson and Naruto end up in the same dimension somehow. Ben could use his Omnitrix to transform into various aliens to fight alongside Naruto against a common enemy like a powerful ninja - alien hybrid villain. They would learn from each other's skills. Ben might learn about chakra control from Naruto, and Naruto could be amazed by the different powers of the aliens.
Well, perhaps the Omnitrix gets corrupted by Hollow energy. Ben has to seek help from the Soul Reapers in the Bleach world to fix it. In the process, he discovers that there are some similarities between the different forms in his Omnitrix and the Zanpakuto spirits. This leads to an exchange of knowledge and power - Ben learning some Soul Reaper techniques and the Soul Reapers getting insights into the alien technology of the Omnitrix.
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.