One plot idea could be Ruby discovers she has werewolf abilities during a mission in a forest. She has to learn to control it while still fighting the Grimm. Another could be that there's a curse in Beacon Academy that turns Ruby into a werewolf, and her friends have to find a way to break the curse without hurting her.
A really interesting plot might be that Ruby is a werewolf by birth, but she's been hiding it from her team. However, when they face a powerful enemy that only a werewolf can defeat, she has to reveal her true nature. Or perhaps there's a parallel world where all the RWBY characters are werewolves, and Ruby has to navigate this new world and its different rules.
One plot idea could be Ruby discovering she has werewolf abilities during a mission. She has to learn to control it while still fighting Grimm. Another could be that in a werewolf form, Ruby has enhanced senses that help her track down a powerful and elusive enemy. Or perhaps she forms a pack with some of her friends who also get werewolf-like powers and they have to protect Vale from a new werewolf - Grimm hybrid threat.
One idea could be Ruby discovering she has some cheetah - like abilities. Maybe she's faster than ever and has to learn how to control this new power. It could add an interesting dynamic to her combat style and how she interacts with her team.
One idea could be that Ruby and Coco team up for a special mission in a dangerous, uncharted part of Remnant. They face new Grimm and have to rely on each other's unique skills to survive. Coco's fashion - sense - related weaponry could combine with Ruby's speed and scythe skills in interesting combat sequences.
A really interesting plot would be if Ruby's Reaper form was somehow connected to an ancient prophecy in the RWBY world. Maybe it was foretold that a Reaper - like figure would either save or doom Remnant. This would put Ruby in a very difficult position, with different factions having different views on her and her new form. Also, a plot where Ruby has to team up with some unlikely allies while in her Reaper form to take on a common threat could be cool. These allies could be former villains who see the greater danger that requires Ruby's power.
One idea could be Ruby having to relearn her combat skills while being deaged. It would add an element of struggle and growth as she tries to regain her former abilities. Maybe she forms new friendships with characters she already knew in a different way because of her new, younger perspective.
A third plot idea could be Ruby having to face an ancient vampire threat that is related to her new condition. Maybe she discovers that she was turned into a vampire as part of a bigger plan by an evil vampire lord, and she has to stop this villain while also coming to terms with her own vampirism. This could involve her learning about vampire lore and using it to her advantage.
One plot idea could be Ruby trying to redeem Torchwick. Since he's a villain in the original series, Ruby could see some good in him and attempt to turn him to the side of good. This would involve a lot of character development for both of them as they interact and Ruby tries to break through Torchwick's tough exterior.
One interesting plot idea could be Ruby exploring a new and dangerous part of the RWBY universe within the Nebula. Maybe she discovers a long - lost relic that has the power to either save or destroy Remnant. Another idea is Ruby meeting alternate versions of her friends in the Nebula and having to figure out how to get back to her own reality while dealing with these new personalities. A third idea could be Ruby facing a powerful new enemy in the Nebula that forces her to develop new skills and strategies that she then brings back to her team in Remnant.
Another plot could be that the wolf pack is actually a group of transformed Huntsmen and Huntresses who were cursed. Ruby, with her pure heart and determination, tries to find a way to break the curse while also dealing with the challenges of keeping their secret from the rest of the RWBY world. She could face internal struggles within the pack as some members are more accepting of their new forms than others.
One plot idea could be Ruby and Winter teaming up for a special mission. Maybe they have to infiltrate an enemy stronghold. Ruby's speed and Crescent Rose could be combined with Winter's ice powers and combat skills for some epic fight scenes.