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.
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 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.
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.
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.
Perhaps there could be a plot where Ruby has to keep her status as a Maiden a secret from her friends at first. This could lead to some interesting misunderstandings and internal conflict as she tries to protect them without revealing the truth.
One interesting plotline could be the exploration of the God of Light's past and his relationship with other deities in the RWBY universe. Maybe how he came to be and what his true intentions are for the world of Remnant.
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.
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.
Another plotline might involve RWBY characters interacting with Earth's superheroes if they exist in the fanfiction. They could team up or have misunderstandings and battles.