Cinder Fall could also be an interesting choice. Given her already somewhat morally ambiguous nature, if she were turned into a Grimm, it could be a really dark turn for her character. She could become an even more formidable foe, using her new Grimm abilities along with her existing combat skills and strategic mind. And it would be fascinating to see how the other characters in RWBY react to her new form.
I think Jaune could be a likely candidate. He's a main character with a lot of development potential. His insecurities and his strong will to become a better Huntsman could be twisted if he were turned into a Grimm. It would create a great internal conflict as he battles his new nature.
One idea could be a character who is turned into a Grimm due to a failed experiment. They then have to struggle with their new form and inner Grimm instincts while still having memories of their human life. Another could be a human turned Grimm as a result of a dark magic spell, and they try to find a way to reverse it while being hunted by other Huntsmen and Grimm alike.
Some characters might be very distrustful. After all, Grimm are seen as pure evil in the RWBY world. They would be cautious around the Grimm - turned - human, constantly watching for any signs of the old Grimm nature to resurface. For instance, Ruby might be hesitant but also curious. Her kind nature might make her want to give the new human a chance, but her experience with Grimm would keep her on guard.
One creative plot twist could be that the once - Grimm - turned - human has memories of their Grimm past. This could lead to internal struggles as they try to come to terms with the evil they once represented. For example, they might have nightmares about their former life as a Grimm and be haunted by the actions they took. Another twist could be that they have special powers related to their Grimm origin. Maybe they can still communicate with Grimm in a way, but use this ability to protect humans instead. It adds an interesting dynamic to the story.
It could be an original character. For example, a young girl who discovers she is half - Grimm after an accident. She has to hide her true nature from others while trying to understand her powers.
It could be a rogue Huntsman who wants to use Ruby's power to control Grimm for his own evil purposes. He might believe that with the Grimm under his control, he can rule over Remnant. He would then be constantly trying to kidnap Ruby or figure out the secret behind her power, and would oppose team RWBY at every turn.
A magical artifact might be the source. For example, a long - forgotten amulet that was created by an ancient civilization with the power to influence Grimm behavior. If a character discovers this amulet and figures out how to use it, they can gain control over the Grimm. This could involve a complex process of deciphering the amulet's inscriptions and learning the proper incantations or rituals associated with it.
There's also a fanfic that delves into the idea of humans being able to control Grimm. In this fanfic, a group of special humans have the power to communicate and direct Grimm. It shows how this new ability changes the power dynamics in the RWBY world and the moral dilemmas that come with it. For example, should they use the Grimm for good or will the power corrupt them?
Definitely Yang Xiao Long. Her fiery personality and her brawler - like fighting style can create some intense scenes when she crosses paths with the Grimm Reaper. Professor Ozpin is another one. His deep knowledge of the RWBY world and his mysterious past can add layers to a fanfiction involving the Grimm Reaper. Also, Cinder Fall. Her evil plans and her power - hungry nature can make for some great conflict when the Grimm Reaper is in the picture.
Well, it could be that Naruto somehow gets transported to the RWBY universe and has to deal with the Grimm. Maybe he uses his ninja skills to fight them off. For example, his shadow clone jutsu could be used to confuse the Grimm while he attacks from another angle.
Magic could be one way. In the RWBY universe, there could be some form of ancient magic that can change the nature of a Grimm. A character might discover a spell or a magical artifact that has the power to make a Grimm friendly. They would then have to figure out how to use it properly without any negative side effects.