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 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 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 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 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.
One plot idea could be that Gwen and Lemon have to team up to rescue Ben from an otherworldly threat. They start off not getting along well due to their different personalities, but as they face various challenges during the rescue mission, they learn to understand and respect each other. Another idea is a time - travel story where they go back in time and accidentally change something in Ben's past, and then have to figure out how to fix it before it causes major disruptions in the present. A third plot could be that a new villain targets Gwen and Lemon specifically because of their connection to Ben, and they have to use their combined powers and wits to defeat the villain.
Well, a plot might involve a device in the Pokemon world that can temporarily give trainers the ability to transform like Ben. But it has a side - effect of attracting some of Ben's most dangerous foes. So, the trainers and Ben must work together to fight off these threats while also dealing with the chaos that the newfound powers are causing in the Pokemon world. They could also learn from each other's fighting styles and techniques during the battles.
One idea could be that the Nemetrix starts malfunctioning and gives the alien predators it controls some of Ben's Omnitrix powers. This would create a whole new set of challenges for Ben as he has to deal with enemies that are a strange mix of the two technologies.