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.
A cool plot could be that there's a villain who can manipulate time and space, and Ruby and Scarlet Spider have to constantly adapt to the changing battlefields. Ruby's semblance of speed could be enhanced or hindered in strange ways, and Scarlet Spider would have to rely on his instincts more. They'd have to figure out how to work together despite the chaos and the differences in their fighting styles.
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.
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.
Ruby is very optimistic and determined. Scarlet Spider is often more brooding and has a sense of street - smart cynicism. Ruby's positive attitude could start to chip away at Scarlet Spider's more negative outlook over time. For example, in a tough situation, Ruby would be the one to see the silver lining and encourage Scarlet Spider to keep fighting.
One common theme is the exploration of Ruby's powers. In these fanfictions, the spider element often adds a new dimension to her abilities, like enhanced agility or the ability to create spider - like constructs. Another theme is the idea of Ruby facing new and unique challenges. The spider - related scenarios are often used to put her in situations she hasn't encountered before in the main RWBY series.
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.
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.
Well, in some fanfictions, a male Ruby might be from a different background than the original Ruby. This could lead to an interesting dynamic with Weiss. For example, he could be from a more noble family, which causes some initial friction with Weiss, who is also from a noble background. As they work through their differences, a White Rose relationship begins to develop. It could involve them facing off against a new type of Grimm that only male Ruby's special skills can help defeat, bringing them closer together.