Reincarnating as Midoriya but with some changes in his quirk manifestation could be fascinating. For instance, what if instead of getting One for All in the normal way, the reincarnated Midoriya discovers a different way to unlock and use the quirk? This could lead to a whole new set of adventures and challenges, and also a different dynamic with the other characters.
Reincarnating as Midoriya Izuku before he gets One For All would be a great choice. You could then explore different ways he could have become a hero without inheriting that quirk. He could develop his own unique fighting style based on his intelligence and analysis skills. Also, reincarnating as Todoroki Shoto and changing his relationship with his father from the start, perhaps leading to a more harmonious family and a different kind of hero development for Todoroki.
Another plot could be reincarnating as a student in a different class than the main characters. The protagonist then has to work hard to prove themselves and make their own mark in the hero school, while also interacting with the well - known characters in new and unexpected ways. For example, they might form an alliance with a character who was originally an antagonist in the series.
All Might could also be an interesting choice. If someone was reincarnated as a younger All Might, they could change some of the events that led to his decline in health. They could also learn from his mistakes and become an even more powerful and long - lasting symbol of peace.
Midoriya and Naruto would be an interesting pairing. Midoriya's determination to become a great hero is similar to Naruto's drive to become Hokage. They could learn a lot from each other. Another would be Bakugo and Vegeta. Both are hot - headed and extremely powerful. They might start off as rivals but could end up respecting each other. And then there's Uraraka and Nami. Nami's skills with money and navigation could be useful to Uraraka, while Uraraka's quirk could be handy in combat situations for Nami.
I think being reincarnated as Loki would be super interesting. He's such a complex character with his own motives and schemes. You could take over Loki's life at a certain point and use your modern - day knowledge to either further his plans in a different way or completely change his path. Maybe you could prevent him from getting into so much trouble with Thor and the Avengers, or use his shapeshifting abilities in ways that no one in the Marvel universe has ever seen before.
Another plot could be that the reincarnated person has a unique quirk from their past life that is very different from the quirks in the My Hero Academia universe. This could lead to them being both feared and respected. They might have to go through a lot of training to control it and prove that they are on the side of good.
One idea could be reincarnating as a minor villain and trying to reform. For example, using knowledge of the future to avoid getting caught up in big evil plans. Another could be reincarnating as a quirkless student but finding a unique way to be a hero, like developing amazing combat skills without a quirk. Maybe reincarnating as a side character and becoming the mentor to the main heroes, guiding them with knowledge from the original story.
Reincarnating as Carlisle Cullen could be interesting. You could explore his past more deeply and maybe change some of his decisions regarding the vampire lifestyle. Another option would be to reincarnate as Rosalie Hale. You could show her softer side and how she copes with her past traumas in a different way.
It could be a new student at U.A. High School. Maybe someone who was a hardcore gamer in their previous life and now has to adapt their gaming skills to the hero world. They would have unique ways of approaching hero training and battles.
Being reincarnated into Master Roshi would be an unexpected but fascinating choice. With the knowledge of modern times, the reincarnated person could introduce new training techniques to Goku and Krillin from the very beginning. They could also use Roshi's pervy side in a more comical way to create some light - hearted moments while still being a respected martial arts master.