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.
A cool plot could be that the Green Lanterns are investigating some strange energy signatures that turn out to be related to the Omnitrix. Ben gets caught in the middle of this investigation. As they interact, Ben discovers new ways to use his alien forms in combination with the Green Lantern's power, and the Green Lanterns gain a new respect for Ben's abilities.
Vilgax from Ben 10 could be an interesting character to include. If he somehow gets his hands on a Green Lantern ring or the technology related to it, he could become an even more formidable foe. This would force Ben and the Green Lanterns to team up against a common enemy who has combined the power of both universes in a dangerous way.
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.
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.
A cool plot could be that a dimensional rift opens up and pulls some of the Young Justice characters into the Ben 10 universe. They then have to find a way to get back home. Along the way, they meet Ben and join forces with him. Ben shows them around his world, and they help him deal with some local threats. For instance, Aqualad could use his water - based powers in conjunction with Ben's Four Arms in a fight against a group of robotic enemies that are causing chaos in a city near Bellwood.
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.