One idea could be a story where Ruby and Scarlet team up to face a new, powerful Grimm that has unique abilities. Maybe it can manipulate time, adding an interesting twist to their battles.
Maybe a plot where Ruby discovers a hidden part of her family's past that links her to the Scarlet Spider in some way. This could lead to her having to learn new abilities or face a moral dilemma as she gets involved with the Scarlet Spider's more gritty and morally ambiguous world compared to the more straightforward good - vs - evil of RWBY.
One idea could be a story where Neo and Ruby are forced to team up during a special mission in a mysterious new area. They start off distrusting each other but gradually learn to work together as they face various challenges.
In some 'rwby dark ruby fanfiction', there might be the idea of Ruby being corrupted by an ancient evil force. This could lead to her having to fight against her own nature as she is slowly turned into something she hates. It can also show how her friends try to save her from this dark path, which makes for a really engaging story.
There are fanfictions where Scarlet Rose and Ruby Rose are transported to an alternate dimension. In this dimension, they have to find a way back while also dealing with the different rules and threats there. It could involve them meeting alternate versions of their friends and foes.
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.
One idea could be a fanfiction where Ruby discovers a new type of Dust with unique powers. This could lead her on an adventure to find out more about its origin and how it can be used to protect Remnant. Another idea is a story that focuses on Ruby's training with a mysterious mentor who teaches her new combat techniques.
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.