Well, without having read it directly, it could potentially be about RWBY characters attacking this 'Ben' in a scenario that the author of the fanfiction has devised. Maybe there's some sort of power struggle or personal vendetta within the RWBY universe combined with the addition of this 'Ben' character. It could also be a misinterpretation of a more complex relationship that the fanfiction is exploring.
Well, from what we can gather, it seems the RWBY characters will be important. Maybe Ruby, Weiss, Blake, and Yang could be involved as they are the main characters in RWBY. And then there's this 'Ben' character. It's possible that the story focuses on their interactions, especially with the RWBY characters being in an attacking role towards Ben. However, it could also introduce other secondary characters from the RWBY universe or even create new ones for the fanfiction's plot.
Another theme is the building of relationships within the harem. It's about how Ben, coming from his own world, forms bonds with the different and diverse characters from RWBY. This includes themes of friendship turning into something more, as well as the challenges that come with having multiple relationships. There's also often the theme of Ben adapting to the RWBY world's culture and society while still maintaining his own identity.
The characters could interact through a portal that connects their two universes. For instance, Ben could be exploring different dimensions and stumble upon Remnant. Then he meets the RWBY team, and they start to exchange knowledge about their respective worlds.
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 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.
I think the interaction between Ruby and Ben would be really interesting. Ruby is very energetic and has a strong sense of justice. Ben, with his experience of using the Omnitrix to save the day, could share some of his stories with her. They could also have some friendly competitions regarding their combat skills, like who can take down a group of Grimm faster, with Ruby using her Crescent Rose and Ben using one of his alien forms.
Ben and Ruby would be an interesting pairing. Ruby is young and full of energy, always eager to learn and improve. Ben, with his experience using the Omnitrix, could teach her a lot about adapting to different situations. And Ruby could show Ben the importance of teamwork in her own unique way.
Another one could be 'Ben's Adventure in RWBY's Harem'. It focuses more on the harem aspect while still maintaining the essence of the Ben 10 and RWBY universes. The story has a lot of comical moments as Ben tries to navigate his new relationships within the RWBY world.