Maybe Ben could accidentally end up in Remnant and his Omnitrix starts acting strange due to the different energy sources there. The RWBY characters could initially be suspicious of him but then realize he can be an ally in their fight against Grimm. They could then work together to figure out how to get Ben back to his own universe while also dealing with the local threats.
Another plot could involve a villain from the RWBY world getting hold of some Ben 10 technology. This could give them an unexpected power boost, and the heroes from both worlds have to work together to retrieve the technology and defeat the enhanced villain. They would have to combine their unique skills and powers, like Ruby's speed and scythe - wielding abilities with Ben's various alien forms and powers.
One idea could be that the characters from RWBY and Ben 10 universes team up to fight a common enemy that threatens both their worlds. For example, a powerful alien race in the Ben 10 world that has somehow found a way to access Remnant (the RWBY world) and is causing chaos.
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.
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.
Maybe in the fanfiction, Attea is in trouble on her home planet and Ben, being the hero he is, goes to help her. Along the way, they discover a long - lost secret about their two species that changes their relationship forever. This could involve some cool battles and new alien forms for Ben to use.
One idea could be that Ben uses his Omnitrix to transform into a Pokemon - like form. For example, he could turn into a fire - type alien that has the abilities of a Charizard. Another plot could involve Ben and Ash Ketchum teaming up to stop a powerful villain who is trying to capture all the Pokemon and use the Omnitrix for evil purposes. Maybe there's a device that can fuse Pokemon with Ben's aliens, creating super - powerful hybrids.
One idea could be a crossover where the Miraculous heroes and Ben 10 team up to fight a common enemy. Maybe an alien villain that is immune to the powers of the Miraculous but Ben's alien forms can counter. Another could be a situation where Ladybug's Lucky Charm combines with one of Ben's alien gadgets to create a super - weapon. And a third could be a story where Ben gets akumatized and the Miraculous heroes have to figure out how to save him without using brute force as his alien forms are very powerful.
Perhaps a story where an old enemy of Ben's targets Rook to get to Ben. Ben feels guilty and protective. They go on the run, hiding in different places across the galaxy. Along the way, they meet some interesting characters who help them. In the end, they face the enemy together, with Ben using his Omnitrix and Rook using his upgraded Proto - Tool to defeat the bad guy.
One idea could be a new student arriving at the school with a mysterious power that no one has seen before. This could cause both excitement and suspicion among the other students.
One idea could be a mercenary who is hired by a mysterious client to steal an ancient relic from Beacon Academy. He has to navigate through the various defenses and teams of Huntsmen and Huntresses.
A plot could involve an esports competition where teams from different kingdoms in the RWBY universe compete. The story could explore how the RWBY team overcomes rivalries and strategic differences. They might also have to deal with a mysterious hacker trying to disrupt the tournament.
Well, you could have a story where all the RWBY characters swap their semblances for a day in chibi form. Ruby with Weiss's glyphs, Weiss with Blake's shadow clones, Blake with Yang's strength, and Yang with Ruby's speed. It would lead to some really funny and unexpected situations as they try to go about their normal activities at school. Or perhaps a fanfiction where the chibi RWBY teams are transported to a world made entirely of sweets. They have to resist the temptation to eat everything while also finding a way back home. A third option might be a mystery plot. A valuable item has gone missing in Beacon's chibi - version campus, and the teams have to solve the mystery of who took it and why.