One idea could be Ruby and Blake getting lost in a mysterious forest during a mission in Volume 5. They have to rely on each other's skills, Ruby's speed and Blake's stealth, to find their way out. Another could be that they discover a long - lost relic together and have to protect it from a group of Grimm.
A fanfiction could be written about a school project in Volume 1 that Ruby and Blake are paired up for. At first, they struggle to work together because of their different personalities. But as they research and present their project, they find common ground and start to see each other in a new light, leading to a closer relationship.
A plot idea might be that Ruby and Blake are sisters separated at birth. Ruby was raised in Patch while Blake was in Menagerie. They find out when they both start having strange abilities that are related to their shared heritage. For example, they can communicate telepathically during times of great danger. As they explore this new connection, they also have to deal with the fact that there are some in the world who would see their union as a threat to the status quo. They then have to fight to protect their sisterhood and the world at the same time.
One great fanfiction could be about Ruby and Blake's first meeting in Volume 1. It might explore how Ruby's bubbly personality contrasted with Blake's more reserved nature at that time. Maybe they start off on the wrong foot, with Blake being a bit standoffish and Ruby trying too hard to be friendly. But as they face Grimm together, they begin to understand and respect each other more.
One plot idea could be that Ruby, as a Faunus, has some unique abilities related to her Faunus nature that she and Blake discover together. They go on adventures to understand and control these abilities while also strengthening their relationship.
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.
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 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.
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 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 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.