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.
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.
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.
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.
If Ruby becomes a Grimm in RWBY fanfiction, it could lead to a really interesting plot. Maybe she could be a special type of Grimm that retains some of her memories and skills. This could make her a very unique and powerful entity. Her transformation might also force the other characters to re - evaluate their views on Grimm. Are all Grimm truly just mindless monsters? Ruby's case could prove otherwise.
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.
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.
Perhaps it could be an evil version of one of Ruby's own relatives. Like an alternate version of her father or mother who has been corrupted by some dark force. They would then raise Ruby with their evil values, making her believe that what they do is right and leading her down a dark path.
A common theme is the corruption of Ruby's character. Time travel often serves as the catalyst for her transformation into an evil version of herself. Another theme is the alteration of the RWBY universe's timeline. As Ruby makes different choices in the past, it causes major changes in the present and future of the world.