One possible plot could be that Izuku discovers Rei's hidden strength. Maybe Rei has some untapped quirk potential that Izuku, with his analytical mind, helps her realize. They could team up to face a new threat in the My Hero Academia world that requires both of their unique skills and experiences.
The main characters would obviously be Izuku Midoriya and Rei Todoroki. Izuku is the protagonist of the main series, a quirkless boy who dreams of becoming a hero. Rei is Shoto Todoroki's mother, who has her own complex backstory and powers. In a fanfiction, they would be the central focus, with their interactions and character development driving the story forward.
Well, the plot could start with Todoroki suddenly realizing he has vampiric traits. Maybe he wakes up one day with a craving for blood. He tries to keep it a secret from everyone at the academy, but his performance in hero training starts to be affected. He becomes weaker in the daylight and stronger at night.
In the fanfiction, perhaps Stain trains Izuku because he believes Izuku has the purity of heart to use power in a way that Stain deems correct. Stain could teach Izuku how to analyze opponents better, which would enhance Izuku's ability to strategize in battles. Izuku might then start to question the current hero ranking system during his training. He could grow to be a hero who is more focused on the essence of heroism rather than just fame and recognition, which would make him stand out among his peers and also cause some friction with the more traditional heroes and hero agencies.
Maybe the fanfiction could focus on Eri's growth under Izuku's care. She starts to see herself as a normal girl with a powerful quirk, not just a tool. Izuku helps her with her schoolwork, they cook together, and he tells her stories about All Might. As time passes, Eri becomes more confident and starts to think about how she can use her quirk to help others, just like Izuku does. She might even start to train to become a hero herself one day.
Another possibility is that they could be in a romantic relationship in fanfiction. Momo's calm and intelligent nature might be attracted to Todoroki's strong and stoic personality. They could support each other through the challenges of being heroes - in - training, and their relationship could develop as they share their hopes and fears with each other.
One common theme is the power of friendship. Even as their relationship develops into something more, the foundation of their friendship is always there. They support each other's growth as heroes.
He could get injured in a fight with a stronger opponent. For example, if he faced an enemy with a quirk that could nullify his own ice and fire abilities, he would be at a disadvantage and could easily get hurt.
Another concept is that Todoroki was turned into a vampire by a villain's quirk gone wrong. He then has to find a way to reverse it. He teams up with Midoriya, who is determined to help his friend. They search for a mysterious old hero who might know a cure, all while Todoroki battles his inner vampire nature.
Izuku could develop his own unique fighting style. He combines his knowledge of hero strategies and his physical abilities. Maybe he invents gadgets like Batman, which give him an edge in battles against quirk - users. This would make him stand out as a badass hero in the fanfiction world.
The plot might revolve around a strange phenomenon that links their two universes. Female Izuku and Giorno are brought together and at first are confused. However, they soon start to communicate and find that they are both heroes in their own right. They have to figure out how to get back to their respective worlds while also dealing with the feelings that start to develop between them. This could involve them learning from each other's combat and heroics techniques.