Perhaps Sasuke is mistaken for an enemy at first. His strange powers and appearance raise suspicion. But as the story progresses, it's discovered that he has knowledge of a greater threat that is coming to the RWBY world, something that might be related to his own past experiences. So he teams up with the RWBY characters to prepare for this threat.
One plot idea could be that a character from RWBY gets transported to the world of Overlord. They might have to navigate the power - hungry and often dangerous world, trying to find a way back home while also dealing with the various powerful beings in Overlord. For example, Ruby could end up in the Tomb of Nazarick and have to figure out how to interact with the Floor Guardians without getting killed.
One idea could be exploring a different backstory for one of the main characters. For example, Ruby could have grown up in a more isolated part of Remnant and had to learn combat skills in a very different way. Another could be a time - travel plot where a character goes back in time to change a major event in the RWBY universe. Maybe they try to prevent the fall of Beacon earlier in the story, which would have a huge impact on all the characters and their relationships.
One idea could be a character getting a tattoo that has a hidden power. For example, a RWBY character might get a tattoo that glows when danger is near, like a warning sign. Another plot idea could be a story centered around a tattoo artist in the RWBY universe who creates unique tattoos for the Huntsmen and Huntresses, and each tattoo gives them a special ability or connection to the Dust energy. Maybe a tattoo that allows a character to communicate with Grimm in a strange way, adding an interesting twist to the story.
One idea could be exploring new Grimm species in the RWBY GP fanfic world. Maybe there are Grimm that can blend into the environment in a more advanced way, like turning into man - made structures to ambush Huntsmen and Huntresses. Another plot could revolve around a new character who has a unique weapon design that combines different Dust types in an unorthodox way, and they have to prove themselves in the academy.
One idea could be the young RWBY characters having a mini adventure in the woods near their homes. They might discover a strange creature and try to befriend it. Another could be them having a pretend battle tournament in their backyard, with each using their unique abilities in a more innocent and unrefined way.
One idea could be a choice where the characters have to decide whether to ally with a mysterious new group in Vale. This new group seems to have some unique powers and resources, but their motives are unclear. It could lead to different story arcs depending on the choice made.
One idea could be the RWBY characters teaming up with the Payday gang to steal some valuable Dust from a powerful and corrupt company. They'd have to use their unique skills, like Ruby's speed and Crescent Rose, combined with the Payday crew's heist expertise.
One idea could be that a Forerunner technology is discovered in Remnant. The characters in RWBY then have to figure out how to use or protect it from the villains. Maybe it's a powerful weapon that could change the balance of power in the world.
One idea could be a story where a criminal organization is stealing Dust and the RWBY team has to go undercover to infiltrate it. They might have to disguise themselves and use their combat skills in unexpected ways.
One idea could be to explore an alternate universe where the characters have different semblances. For example, Ruby could have a semblance that allows her to manipulate time briefly instead of her speed. Another plot could focus on a new villain in the game's world who has the power to corrupt semblances, forcing the RWBY team to find a way to counter this new threat without relying solely on their usual powers. Also, a story about the backstories of the minor characters in the 'rwby the game' could be really engaging.