A cool plot could involve the characters from Frozen getting stuck in the RWBY world and having to learn how to fight Grimm. Anna might be really eager to learn combat skills, while Elsa is more focused on controlling her powers in this new and dangerous environment.
Some interesting Frozen 2 fanfiction plot ideas could revolve around the idea of a magical item. For example, there could be a magical crystal hidden deep in the forest that has the power to either save or destroy both the forest and Arendelle. Elsa and her friends embark on a quest to find this crystal before it falls into the wrong hands. Another idea is that a dark force from outside the known world tries to invade the forest and Arendelle, and the characters have to unite not only the people of Arendelle and the forest but also other magical beings to fight against it. Also, a plot could be centered around a prophecy that was never fully revealed in the movie, and Elsa and Anna must figure out what it means for their future.
Another plot could involve a RWBY character ending up in the Naruto universe. Say, Weiss gets sent to Konoha. She might initially struggle with the different culture and the use of chakra, but eventually she could use her Glyphs in unique ways by combining them with chakra. This could also lead to some interesting interactions between her and the ninja students there.
One idea could be that Gilgamesh, with his vast power and arrogance, arrives in the RWBY world. He might initially see the Huntsmen and Huntresses as beneath him but then becomes intrigued by Ruby's unique silver - eyed power. Another could be a scenario where Gilgamesh allies with a villain in RWBY, like Cinder, and together they try to reshape Remnant according to their desires.
One plot idea could be that Elsa from Frozen somehow ends up in the Fallout wasteland. Her ice powers could be both a hindrance and a help. For example, she could use her ice to create shelters in the harsh environment, but her powers might also attract unwanted attention from raiders or mutants. Another idea is that the Brotherhood of Steel discovers Elsa and tries to study her powers, thinking it could be some sort of advanced technology. A third plot could involve Anna coming to the Fallout world to find Elsa and they team up with some of the more friendly factions in the game to survive and try to find a way back home.
One plot idea could be Ruby trying to redeem Torchwick. Since he's a villain in the original series, Ruby could see some good in him and attempt to turn him to the side of good. This would involve a lot of character development for both of them as they interact and Ruby tries to break through Torchwick's tough exterior.
Perhaps Salem discovers a way to use Naruto's chakra for her own evil purposes. So, Naruto has to find a way to stop her while also dealing with the new and unfamiliar world of RWBY. He might learn some new skills from the RWBY characters along the way.
One interesting plot idea could be Ruby exploring a new and dangerous part of the RWBY universe within the Nebula. Maybe she discovers a long - lost relic that has the power to either save or destroy Remnant. Another idea is Ruby meeting alternate versions of her friends in the Nebula and having to figure out how to get back to her own reality while dealing with these new personalities. A third idea could be Ruby facing a powerful new enemy in the Nebula that forces her to develop new skills and strategies that she then brings back to her team in Remnant.
One idea could be that Pyrrha and Glynda team up to train a new group of students at Beacon Academy. They face challenges in teaching different fighting styles and personalities. Another could be a story where they go on a mission together to an ancient, magical place and have to rely on each other's skills to survive. Maybe there could be a plot where they discover a long - lost secret about the history of Remnant that only they can figure out how to deal with.
One idea could be that the characters from RWBY gain the ability to use Ki like in Dragonball Z. For example, Ruby could use her speed along with Ki blasts in a new and powerful way. Another plot could involve the villains from both universes teaming up, forcing the heroes to combine their skills and powers. Or perhaps there could be a tournament in the style of the World Martial Arts Tournament where characters from both RWBY and Dragonball Z participate.
Another plot could involve a magic - based tournament where teams from both Fairy Tail and RWBY participate. This would allow for battles between different characters, like Natsu from Fairy Tail facing off against Yang from RWBY. The different fighting styles and power sources would make for exciting match - ups and also a chance for the characters to learn from each other.