Perhaps there could be a crossover where the Akumas from the Ladybug world start appearing in the My Hero Academia world. The heroes in My Hero Academia would be really confused by these strange new villains. Ladybug and Chat Noir arrive to help. But they soon realize that they need to work with the local heroes to stop the Akumas. This would create a great opportunity for character development as they all learn from each other's fighting styles and ways of thinking.
One idea could be that Ladybug's Miraculous powers are somehow connected to a Quirk in the My Hero Academia world. For example, maybe her Lucky Charm ability could be seen as a very unique and unpredictable Quirk. Another plot could involve Ladybug and a My Hero Academia character teaming up to fight a common enemy that is causing chaos in both their worlds. This would require them to learn about each other's powers and work together in a way that combines the best of both universes.
One idea could be that Naruto characters are transported to the My Hero Academia world. Naruto himself could be seen as having a very unique quirk, like his chakra manipulation could be considered a superpower. They could team up with the students of UA to fight against a common enemy, like a group of villains who are after the secrets of chakra. Another plot could involve the My Hero Academia characters traveling to the Naruto world and learning about ninjutsu and how it differs from their quirks.
One idea could be a crossover with the Harry Potter universe. Imagine Deku going to Hogwarts and learning magic while also using his Quirk. He could make friends with Harry and they could learn from each other's skills. Another idea is a crossover with the Avengers. The heroes from My Hero Academia could team up with the Avengers to fight a common, super - powerful enemy. Maybe All Might and Captain America could have an interesting conversation about leadership and heroism.
One idea could be to explore the backstory of a minor character. For example, what was the life of a random student in Class 1 - B like before coming to U.A. High? Another could be a 'what if' scenario, like what if Izuku Midoriya didn't get One For All. Maybe he'd develop his own unique quirk through sheer determination and hard work. Or a fanfic could focus on the villains' side, showing how they came to be and their motives in more depth.
A great crossover idea is with the Dragon Ball series. The powerful Saiyans could interact with the My Hero Academia characters. Goku could train the students at UA to reach new levels of power. Also, a crossover with the Attack on Titan world would be intense. The heroes would have to deal with Titans in a whole new way and use their quirks in that battle - filled environment.
Well, perhaps a plot where Genos follows Saitama into the My Hero Academia world. Genos could enroll in UA as well and show off his advanced technology - based powers. He might get into a competition with Bakugo over who is more powerful. Meanwhile, Saitama is just casually wandering around, accidentally solving problems and leaving the students and heroes in awe of his effortless strength.
Maybe there could be a quirk - semblance exchange. A character from My Hero Academia could temporarily gain a RWBY character's semblance and vice versa. This would lead to some really cool and unexpected battle combinations. Like Deku having Ruby's speed semblance for a while and using it in a creative way during a fight.
A great plot might be a training exchange. The heroes from My Hero Academia go to the DC universe to train with Superman and Batman. They learn different combat and strategic techniques. In return, they teach the DC heroes about quirks. This could lead to some really interesting character interactions. For instance, Bakugo might have some intense sparring sessions with Wonder Woman, and both would learn from each other.
Maybe there could be a plot where a villain from Overwatch steals some of the quirk - enhancing technology from My Hero Academia. This forces the heroes of both universes to work together to get it back. For example, Tracer and Deku could team up. Tracer's time - jumping ability and Deku's One For All would make for an exciting duo in the chase.
A cool plot could involve the sharing of techniques. Sasuke might teach Bakugo some of his sword skills, while Bakugo shows Sasuke how to better control his explosive power without overusing it. Also, the concept of the Ninja Academy in Naruto could be merged with UA High. Students from both could participate in a joint training session where they learn from each other's combat styles and strategies.
One plot idea could be that the heroes from My Hero Academia end up in the Soul Society of Bleach. They could learn about the different spiritual powers there and team up with the Soul Reapers to fight a common enemy, like a powerful Hollows that have somehow invaded both worlds. Another idea is that the Quirks from My Hero Academia start to manifest in the Bleach universe, causing chaos as people with Quirks are not the norm there. And perhaps Ichigo could visit UA Academy and learn about hero training, and even participate in some hero battles.