Well, one plot could be that Ruby's mother faked her death and is actually alive in hiding. Ruby discovers some clues that lead her to believe this and goes on a journey to find her mother. Along the way, she has to face various challenges and enemies who are also after her mother for their own reasons.
A different plot could be that Ruby's mother was from a different kingdom and had a different way of using her powers. In the fanfiction, Ruby starts to have dreams about her mother and these dreams lead her to learn a new and powerful technique that was unique to her mother's side of the family. This then helps Ruby in her battles against the Grimm.
Maybe the story could focus on Ruby being a single mother. She might struggle to balance her duties as a huntress with raising her child. The child could look up to Ruby and want to follow in her footsteps, but Ruby is hesitant because she knows the dangers. She would then have to make decisions about whether to encourage or discourage the child's aspirations.
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.
She could get the eye patch in a training accident. It makes her feel self - conscious at first, but then she realizes it doesn't change who she is. Weiss could be the one to help her come to terms with it by saying something like 'Your strength is not in your eyes, but in your heart'.
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 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.
One plot idea could be that Ruby discovers a hidden stash of nanobots that were created by an ancient civilization in Remnant. These nanobots start to malfunction and cause chaos in Vale. Ruby has to use her skills and Crescent Rose to contain and fix the nanobots. Another idea is that a villain uses nanobots to control the minds of some of Ruby's friends, and she has to find a way to break the control and defeat the villain. A third plot could involve Ruby teaming up with a scientist to study the nanobots and use their unique properties to upgrade her weapon or gain new powers.