Perhaps Izuku's father is a scientist who was working on a top - secret project related to quirks. He might have accidentally given Izuku his quirk through an experiment. This could lead to a story where Izuku discovers the truth about his quirk and his father's role in it, while also dealing with the ethical implications of such an origin for his powers.
Perhaps Izuku is investigating a series of disappearances in the hero community. He has to interview various heroes and villains alike to piece together the clues. He could face many challenges and even some threats along the way, but his determination to solve the case keeps him going.
All Might could be an interesting choice. He has been a major influence on Izuku, and if he were his father, it would add a whole new layer to their relationship. It could explain why All Might took such a special interest in Izuku from the start.
A cool fanfiction concept could be to have a character lose their quirk and then have to find a new way to be a hero. This would show their determination and resourcefulness. Or, you could write about a new villain who has a quirk that specifically targets the heroes' weaknesses, and how the heroes would have to work together to overcome this threat.
Another plot could involve Izuku being in a situation where his harem has to work together to protect him. Maybe they find out about a secret plot against Izuku and combine their unique quirks to keep him safe. This would show the different relationships he has with each of them and how they interact as a group.
A great fanfiction idea might involve Izuku's Oraphin side helping him in unexpected ways during the sports festival. For example, it could give him an edge in the obstacle course by allowing him to understand the movements of the other competitors better. It could also lead to some interesting interactions with his classmates, especially those with animal - like quirks.
One idea could be Batman somehow getting transported to the Boku no Hero Academia universe. He could use his detective skills and gadgets to help the heroes there. For example, he might be able to analyze the quirks in a way the local heroes haven't thought of before.
Another idea could be alternate universes where the roles are reversed. The heroes are the ones causing trouble and the villains are trying to maintain order. It gives a whole new perspective on the characters and their relationships. In these fanfictions, the character development can be really fascinating as they adapt to these new roles.
You could also do a fanfiction where Tsukauchi discovers a secret ability within himself. This could completely change his role in the hero world and lead to some really interesting character development as he tries to figure out how to use this new power.
One idea could be creating an OC with a unique quirk like the ability to control shadows but with a drawback that it only works during a full moon. Another is an OC who is a hero from a foreign country with a quirk based on their cultural heritage, for example, a quirk related to traditional dance forms that can be used to disorient villains. You could also have an OC who is a villain turned hero and has to deal with the prejudice from both sides.
Another interesting idea is to create a fanfiction where the characters from 'boku no hero academia' visit the Cubicity world and have to adapt to its strange rules and customs. They could face new challenges and form unexpected alliances.