One interesting plotline could be the characters facing a new and powerful Grimm threat that they have to overcome together. This would test their skills and teamwork.
Since I haven't had the chance to read 'rwby by your side by anonymusark fanfiction', it's hard to say precisely. However, in RWBY fanfictions, the title can give some clues. 'By your side' might imply a story of companionship. It could be that a character from RWBY is standing by another character's side through various events. It could be a story full of action and adventure where the characters rely on each other, or it could be more of a character - study on the relationship between two or more characters in the RWBY universe.
A great plotline might involve the deaged characters interacting with their younger selves' memories. Weiss could be haunted by her family's expectations in a new light, as if seeing them for the first time all over again. This could lead to her making different choices and growing in a whole new direction.
One possible plotline could be a character sacrificing their own powers to save their friends from a powerful enemy. For example, Ruby might give up her Silver - eyed Warrior abilities to stop Salem's latest scheme and protect her team.
Well, in some rwby mugetsu fanfiction, there might be a plotline where the characters have to adapt to a new world that combines the two settings. They could struggle with understanding different cultures and magic systems. Maybe Weiss has to learn how to use her Glyphs in a world where the rules of magic are more like those in Mugetsu. This would lead to her character growth as she overcomes these challenges.
One possible plotline could be a character facing public shaming in the RWBY universe, perhaps due to a failed mission or a misjudgment. Another could involve a character being humiliated in a social setting within the school or among their team.
Another plotline could involve the reincarnated character trying to change the fates of the main RWBY characters. They might know about certain events that are going to happen in the future of the RWBY story and work towards preventing tragedies or enhancing the characters' growth in different ways. Maybe they try to save Pyrrha from her doomed fate at the hands of Cinder.
A great plotline could be set in an alternate universe where Ozpin and Glynda are not in their typical roles. They could be ordinary hunters trying to survive in a world that's even more chaotic than the main RWBY universe. This would give a chance to see their characters develop in different ways, without the constraints of their normal positions and responsibilities.
One interesting plotline could be the origin story of the Blood Rose in the RWBY universe. Maybe it's a weapon with a hidden power that the main character discovers.
Perhaps a plot where a dragon rider is from a different dimension and has to adapt to the world of RWBY. They could bring unique skills and knowledge that help the RWBY gang in their battles and also learn a lot from them in return. This could involve cultural exchanges as well as combat strategies.
A plotline could involve a crossover within the RWBY universe. In the 'rwby crimson blizzard fanfiction', characters from different teams could be brought together by the strange and dangerous phenomenon of the crimson blizzard. They might initially clash due to their different fighting styles and personalities, but as they struggle to survive the blizzard and the threats it brings, they learn to work together and discover new aspects of their own abilities.