A plot could involve Izuku Midoriya being transported to the Re:Zero world. He would be shocked by the different rules and magic there. But his heroic nature would lead him to try and help the people in this new world. He could team up with Subaru to face the Witch Cult. Izuku's quick thinking and his One For All quirk would be useful in combat. And through his interaction with the characters in Re:Zero, he might gain new insights about heroism that are different from what he's learned in his own world.
Maybe there could be a plot where some of the quirks in My Hero Academia start to appear in the Re:Zero world. This causes chaos as the people there don't understand these new powers. Emilia could befriend some of the My Hero Academia characters who come to help deal with the situation. For instance, Todoroki's ice quirk could be used to stop a powerful magical beast in the Re:Zero world. And in the process, the characters from both worlds learn about the importance of cooperation and understanding different cultures.
One possible plot could be that Subaru finds himself in the world of My Hero Academia. He might initially be confused by the existence of quirks. Since he has the ability to return by death, he could use this to his advantage in battles against villains. For example, he could keep trying different strategies to defeat a powerful villain like All For One, learning from each death and getting closer to victory each time. He could also form friendships with the students at U.A., sharing his own experiences of struggle and perseverance which would inspire them in return.
Subaru and Izuku would be an interesting pair. Izuku is a born hero with his strong sense of justice and his powerful quirk, while Subaru is someone who has to constantly struggle and rely on his return by death ability. Their different personalities and abilities would create a lot of interesting interactions. Izuku could inspire Subaru to be more confident in his own abilities without relying solely on his special power, and Subaru could show Izuku the value of perseverance in the face of extreme difficulties.
Another plot could involve the Pleiades from Overlord. They end up in Re:Zero and are mistaken for some sort of magical spirits. Subaru tries to enlist their help in his quest to save Emilia. The Pleiades, with their combat skills and unique abilities, could change the power dynamics in the Re:Zero world and also introduce new technologies and magic concepts from Overlord.
One idea could be a crossover with 'One Piece'. The heroes from 'boku no hero academia' could end up on the Grand Line and have to deal with pirates while also using their quirks. Another is a crossover with 'Naruto'. Maybe the students of UA could train with the ninja in Konoha, learning new skills and sharing their quirk knowledge. A third could be a crossover with 'Attack on Titan', where the heroes have to face the Titans and figure out how to use their quirks to defeat these huge threats.
Maybe the plot involves a device created by Stark Industries that malfunctions and creates a portal between the two universes. The heroes from both sides initially have some misunderstandings and fight each other. But then they realize that there is a greater threat. The My Hero Academia students could learn a lot from the Avengers' teamwork and fighting styles, while the Avengers are impressed by the creativity of the quirks. They then join forces to stop a group of villains who are trying to use the portal to invade both worlds.
One idea could be that the Pokemon have Quirks. For example, Pikachu could have an electric - based Quirk that is even more powerful than its normal electric attacks. The heroes in training from My Hero Academia could team up with these special Pokemon to fight against villains who are also using Pokemon with dangerous Quirks. It would be a really cool mix of the two universes.
Another possibility is a crossover with 'My Hero Academia'. Subaru could be a student at U.A. High School. His Return by Death power would be like a quirk. He might struggle at first to control it and fit in, but eventually use it to save his classmates from villains, in a way that surprises everyone with his growth and resourcefulness.
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.
A plot twist could be that the Quirk system and Stands are somehow related. Maybe there was an ancient event that split the power into two forms. This would open up a whole new exploration of the lore in both universes. Characters would have to reevaluate their understanding of their own powers and how they interact with those from the other universe.
A possible plot is that the Reapers from Mass Effect start invading the world of My Hero Academia. The heroes would have to use their quirks in new and creative ways to fight off these giant mechanical threats. Maybe All Might could inspire the Mass Effect heroes with his unwavering spirit, while Shepard could teach the My Hero Academia characters about advanced military tactics.
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.