Well, one possibility is that Ruby could be influenced by a powerful and evil relic. Maybe she discovers a dark side of the relic's power and becomes corrupted by it, gradually turning evil as she starts to value power over her friends and the greater good.
One possible plot could be Ruby being betrayed by her closest friends. Maybe she discovers a dark secret about the Huntsman academies that makes her question everything she has ever believed in. This disillusionment could start her on the path to evil as she feels that the system she has dedicated her life to is corrupt.
Well, it could involve Ruby being influenced by an evil force in this fanfic. Maybe she's taught dark magic or has her moral compass twisted. For example, she might start seeing the world in a more cynical way and use her powers for less noble purposes.
The main antagonist could also be a new character created specifically for the fanfic. This character could be an ancient evil that has been waiting for an opportunity to corrupt a powerful Huntress like Ruby. They would then use Ruby's newfound evil nature to further their own dark plans, and Weiss, Blake, and Yang would have to face both Ruby and this new threat.
If Ruby turns evil, she could become her own antagonist in a way. Her former self, the Ruby that was good, could be seen as what she is now fighting against. Her friends would also become antagonists as they try to stop her from causing more harm.
If Ruby turns into a Grimm in RWBY fanfiction, it could be a really dark turn. Her silver eyes, which are usually a weapon against Grimm, might change. She might start to be attracted to the Grimm way of life. But on the other hand, her friends would never give up on her. They would try to find a way to reverse the transformation. Maybe they would search for some ancient magic or technology. It would be a story full of conflict, both internal for Ruby and external between her friends and the Grimm world.
It could be that Ruby has to deal with the consequences of a major decision she made in the past. This causes her to question her leadership skills and she embarks on a journey of self - discovery to become a better leader for her team.
The plot could involve Ruby finding a balloon that has strange powers. For instance, it might allow her to fly, which is something she doesn't normally do without her scythe's recoil. This new ability could change the way she approaches battles and exploration in the RWBY world.
Another plot could be that he is exposed to some sort of mind - controlling substance or technology. The enemy could be using it to turn him against Gotham. Under this influence, he starts to see his friends as foes. He could begin to attack the Bat - family and other superhero allies. His actions would be erratic and violent, and the city would be in chaos as they try to figure out what happened to their hero and how to stop him.
Well, one possibility could be that Ruby gains a whole new level of guidance. Ozpin might introduce her to secrets of the world she never knew. Maybe he trains her in unique ways, using his vast knowledge and experience, which helps Ruby become an even more powerful huntress in the long run.
One possible plot could be that Ruby discovers Yamato in a strange new world or dimension within the RWBY universe. She has to figure out how to use it or control it while also facing the challenges that come with it, like other Grimm or villains being attracted to the power of Yamato.