A plot could involve Ruby being deaged and getting lost in a different part of Remnant. She has to rely on her instincts and the kindness of strangers to find her way back. This would show her vulnerability and also her ability to adapt even in a deaged state. She might also discover new things about Remnant that she didn't know before, which could change her view of the world when she returns to her normal age.
Perhaps in the fanfiction, Ruby being deaged causes her to lose some of her memories related to the more complex parts of her life, like her battles against Salem. As she tries to piece together her past, she starts to question things she once took for granted. This could lead to her making different choices when she finally regains her memories, and it would also be a great way to explore her character in a deeper way.
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.
One idea could be Ruby adjusting to her new cyborg body and the enhanced abilities it gives her, like super - strength or advanced weaponry integration. Another could be her facing discrimination from some in the RWBY world who fear or mistrust cyborgs.
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 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 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.
Weiss would probably be very protective at first. She has always had a bit of a mother - hen side to her, and seeing Ruby in a more vulnerable, deaged state would bring that out. She would likely try to keep Ruby out of trouble and make sure she was taking care of herself.
One plot idea could be that Ruby discovers a hidden connection between her Silver - eyed powers and Ozma's ancient magic. Maybe she has visions that lead her to an old relic related to Ozma. Another could be that they team up to fight a new, powerful Grimm threat that is immune to normal Huntsman attacks, but Ruby's unique abilities combined with Ozma's knowledge can defeat it.
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.