Well, it could be about Natsu finding himself in the RWBY universe and having to adapt to the different combat styles there. Maybe he teams up with the RWBY characters to fight a common enemy.
Definitely Ruby Rose. Since she's the main protagonist in RWBY, it's likely that Natsu would cross paths with her first. Their personalities are quite different, with Ruby being more innocent and Natsu being more brash, so their interactions would be interesting.
It could be Zeref. If Natsu goes back in time, Zeref might have different plans or be in a more powerful position at that time. He could be the main obstacle for Natsu to overcome, especially if Natsu is trying to change things related to the dark magic that Zeref wields.
One interesting plotline could be Natsu going back in time and accidentally changing something that has a huge impact on the future of Fairy Tail. For example, he might prevent a key member from joining the guild, and then he has to figure out how to fix it.
Well, in 'rwby neo goes to beacon fanfiction', Neo could be exploring the Beacon Academy. She might be trying to find a place for herself among the students there. It's possible that she'll cross paths with the main RWBY characters and have some interesting interactions with them. For example, she could team up with them in a mission or have a friendly sparring session.
In the 'rwby cfvy goes with rnjr fanfiction', the main characters are the members of both teams. For CFVY, Velvet is an important character as she has her own unique Semblance. Coco is also a main character with her stylish combat style. For RNJR, Ruby is the main character as she is the leader. Jaune is also important as he has grown a lot in the series. And Nora and Ren add their own unique personalities and abilities to the mix. All of these characters will likely interact a great deal in the fanfiction.
Well, it could be about the adventures that the RWBY characters from CFVY and RNJR go on together. Maybe they are on a mission to fight some new Grimm threats. They might discover new abilities or form new friendships along the way.
Well, obviously, the main characters would be the members of Team RWBY, which are Ruby, Weiss, Blake, and Yang. They are the central focus as they journey to Menagerie.