One idea could be that Cinder realizes the true cost of her actions after seeing the destruction she has caused on a personal level. For example, she might encounter a family that has been torn apart because of the events she set in motion. This could be the start of her journey towards redemption.
One idea could be Cinder being transported to an alternate universe within the RWBY world where the roles are reversed. For example, she could find herself in a world where she is a huntress in training, facing the same challenges as the main characters but from a different perspective. Another plot could involve a team - up between Cinder and a group of RWBY characters against a common, greater enemy that threatens both their goals and the world. Maybe a new, powerful Grimm that has abilities they've never seen before.
A more emotional plot could be that they discover they have a shared past that they were unaware of. Maybe they both had a connection to a certain character who was important in their lives before they became who they are now. This revelation makes them question their own paths and they start to see each other in a new light, leading to a tentative alliance and exploration of their similarities and differences.
One idea could be a fanfic where Cinder forms an unexpected alliance with a Beowulf. Maybe she's on a mission alone and gets surrounded by enemies, and a Beowulf, which is usually a foe, decides to help her for its own mysterious reasons. This could lead to a whole new power dynamic and a strange friendship or partnership.
One interesting concept could be the exploration of their opposing natures. Ruby is often seen as pure - hearted and full of hope, while Cinder is more devious and power - hungry. In fanfiction, this contrast can lead to some intense character - driven stories where they either learn from each other or clash in more complex ways than in the original series.
Another idea is a time - travel fanfic. Cinder travels back in time and meets a younger Ruby. She could try to manipulate Ruby into following a different path, but Ruby's strong will and sense of justice might resist. This would create a lot of conflict and interesting character dynamics as Cinder tries to change the future and Ruby tries to protect it.
One possible storyline could be a redemption arc for Cinder through her relationship with Pyrrha. Maybe in an alternate universe, Cinder realizes the error of her ways and Pyrrha helps her to change. They could start as enemies but gradually come to understand each other.
For Cinder's redemption arc to be believable, her motivation for change needs to be well - developed. It can't just be a sudden change of heart. Maybe she discovers some long - lost family or a part of her past that makes her question her actions as a villain.
One idea could be a crossover where Cinder somehow gets transported to the Naruto universe. She might initially try to use her powers to gain control, but then realizes she has to adapt to the new chakra - based powers. Meanwhile, RWBY characters could also end up in the Naruto world in search of Cinder and end up forming unexpected alliances with Naruto and his friends.
One interesting plot could be Ozpin secretly training to counter Cinder's powers. He might be gathering ancient knowledge and artifacts to gain an edge.
One interesting story could be where Cinder from RWBY somehow ends up in the Halo universe and has to adapt to its technology and combat styles. Maybe she teams up with some Halo characters to fight a common enemy.