One idea could be that the Lucky Girl discovers a new alien form that Ben doesn't know about yet. She has to figure out how to use its powers to save the day. Maybe it has the power to manipulate time in a small area, and she uses it to stop a villain from stealing an important artifact.
A plot could involve the Lucky Girl getting accidentally caught in one of Ben's transformations and they end up merged in a way. They have to learn to work together while sharing a body. This could lead to some comical situations as they try to control the alien form and also some deep moments as they understand each other's perspectives better.
One interesting idea could be how she would handle the different alien forms. Since Ben 10 has a variety of powerful alien forms, as a girl, she might approach using them in a more empathetic way. For example, when using the Four Arms form, instead of just brute strength, she could use it to protect the innocent more gently.
One idea could be that as a girl, Ben has to face different social challenges while still using the Omnitrix to save the day. For example, some might underestimate her because of her gender, but she proves them wrong with her heroic deeds.
One plot idea could be that Ben turns into a girl and has to deal with the different social expectations. For example, his friends might treat him/her differently at first. Another could be that the Omnitrix malfunctions further and only allows female forms, so Ben has to learn to master these new forms in battle. Maybe there could also be a story where Ben as a girl gets caught up in a situation where only a girl can save the day, like infiltrating an all - female villain group.
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.
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.
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.
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 Tennyson finds himself in a parallel dimension where the Omnitrix has transformed all the female characters into monster girls. He has to figure out how to change them back while also dealing with the new powers and personalities of these monster girls. Another plot could involve a group of evil forces trying to use the monster girl situation to take over the world, and Ben has to team up with some of the friendly monster girls to stop them.
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.
A fanfiction plot could be about a new threat that is specifically targeting the Omnitrix and the Power Rangers' power source. Ben and the Rangers have to figure out how the enemy is doing this and stop them. Along the way, they discover a hidden connection between the two power systems that was unknown before. There could be some character development for both Ben and the Rangers as they struggle to protect what they hold dear.