Perhaps the Sharingan ability from Naruto could be passed on to a character in MHA, and they have to deal with the new power and the consequences that come with it. In reverse, a quirk could be transferred to a Naruto character. This would create a situation where the characters have to figure out how to use these new powers in their own worlds. It could also lead to some interesting battles and alliances as they try to master their new abilities.
One idea could be that Naruto characters visit the MHA world and have to adapt to the modern society and different power system. For example, Naruto might be intrigued by how quirks work compared to chakra. Another plot could involve villains from both universes teaming up, forcing the heroes from Naruto and MHA to collaborate. They would need to combine their unique fighting styles and powers to defeat the combined threat.
One idea could be that the Avengers are somehow transported to the MHA universe. They have to deal with quirks while also trying to find a way back home. Maybe they team up with the students at UA to fight a common villain who has a power that combines both advanced technology and a powerful quirk.
Another possibility is a crossover with the world of Naruto. The characters from MHA could participate in the Chunin Exams. Bakugo might be really into showing off his skills in this new and different kind of competition.
One idea could be that the heroes from MHA are transported to the Super Sentai world and have to team up with the Sentai warriors to fight a common enemy that is threatening both universes. Maybe a powerful villain has found a way to cross dimensions and is causing chaos.
A possible idea for a MHA RWBY crossover fanfic is to explore the idea of semblance and Quirk evolution. The characters from both shows could train together and discover new ways to enhance their abilities. For instance, Izuku Midoriya could learn from Ruby about using her Crescent Rose in combat and in turn, show her how to better control her power flow. The cultural differences between the two worlds could also add depth to the story, like how the heroes in MHA are more publicly recognized compared to RWBY, and how the characters react to this disparity.
Maybe Ladybug's Miraculous gets mixed up with an MHA student's Quirk. So the student starts to exhibit powers related to both the Miraculous and their original Quirk. This could lead to some really cool fight scenes where they have to learn to control these new combined powers. They might also face challenges as the other characters in both universes are skeptical about this new power combination.
One idea could be a character going back in time to prevent a major villain's origin. For example, if someone went back to stop All for One from corrupting Tomura Shigaraki as a child. Another plot could involve a hero traveling to the future to see the outcome of their current actions and then coming back to make changes. Maybe a student like Izuku Midoriya goes to a future where the heroes have lost and tries to change the present to avoid that fate.
A cool plot might be that the Hogwarts students attend U.A. High School. Hermione would surely excel in academic subjects there, while Ron could struggle a bit more. And Harry could use his experience with fighting dark magic to help in hero training. They could also introduce magic - based quirks to the MHA world, which would change the dynamic of how heroics are done.
One interesting plot idea could be a crossover where Naruto's senjutsu abilities are combined with another universe's magic system. For example, if he were to cross over into the Harry Potter universe, he could use senjutsu to enhance his magic or even teach the wizards about chakra. Another idea is a crossover with the Dragon Ball universe, where Naruto's senjutsu clashes with the ki energy of the Saiyans. Maybe he has to use senjutsu to face off against a powerful Saiyan who has come to the Naruto world. Or a crossover with the Bleach universe, and Naruto uses senjutsu to interact with the spiritual energy there, perhaps even helping Ichigo in a battle against a powerful Hollow by using his unique senjutsu skills.
One plot idea could be that Naruto characters end up in Ikebukuro. For example, Naruto himself could be searching for a new type of ninja tool that he heard can only be found in Ikebukuro. Meanwhile, the gangs in Durarara start to notice this strange, orange - clad boy with strange powers. Another idea is a character swap where some of the Durarara characters gain ninja abilities and get involved in Naruto - style battles in their own world. Maybe Celty could use her shadow powers in a more ninja - like way, like stealth and quick attacks.
One idea could be that Naruto characters are transported into the Splatoon world. They might initially be confused by the ink-based battles and the unique species. For example, Naruto could mistake an Inkling for a strange ninja from another land and challenge them to a friendly spar, only to find out about the ink shooting mechanism. Another plot could involve the Akatsuki from Naruto trying to steal the valuable ink resources in the Splatoon world for their own purposes. They would face off against the Splatoon heroes who are determined to protect their world's resources.