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.
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.
A cool plot could be that there's a tournament across different anime worlds, and Inuyasha and Naruto are chosen to represent their respective worlds. Along the way, they discover similarities in their pasts, like both having some form of demon - related heritage (Naruto with the Nine - Tails and Inuyasha being a half - demon). They also exchange combat techniques. Naruto teaches Inuyasha how to better use chakra control in his attacks, and Inuyasha shows Naruto some unique sword - fighting skills.
One interesting plot idea could be that Kaguya is reborn in the modern Naruto world. She has no memories of her past at first. Naruto discovers her and senses a strange power within her. As she starts to regain her memories, she struggles between her old self and the new world she's in. Naruto has to decide whether to trust her or not.
Perhaps the nine - tailed fox in Naruto and the Avatar state could have some sort of connection. In the fanfiction, the two powers could interact in unexpected ways. For example, when Naruto enters his nine - tailed fox mode, Aang's Avatar state could be affected, or vice versa. They might have to learn to control these interactions to save both their worlds from a dark force that wants to use this power connection for evil.