Well, you could have a plot where the OC is a healer - type with a quirk that can cure any injury or illness. However, there's a catch - using the quirk drains the OC's own life force. The BNHA characters initially doubt the OC's abilities because they seem too good to be true. As the story progresses, the OC proves their worth during a major battle where many heroes are injured. They have to decide how much of their own life they are willing to sacrifice to save others, and this creates a deep emotional and moral dilemma.
One idea could be that the OC is a new vampire in town. Rebekah takes an interest in them because they seem different from the other vampires she's known. They could go on adventures together, facing other supernatural threats while also exploring their growing relationship.
One idea could be that the male OC is reincarnated as a villain in the BNHA world but has memories of his past life as a hero. So he struggles between his new villainous identity and his old heroic values. Another plot could be that he reincarnates as a quirkless character, like Midoriya was initially, but he uses his past - life knowledge to develop unique combat skills. Or perhaps he reincarnates as a powerful hero's sidekick and has to deal with living in the shadow while trying to prove his own worth.
One idea could be to have characters from Boku no Hero Academia (bnha) and Demon Slayer (kny) participate in a joint training session. For example, Deku from bnha could learn some swordsmanship techniques from Tanjiro of kny. This would create an interesting dynamic as they come from different worlds with different power systems.
One idea could be that the villains release a powerful virus that turns most of the population into mindless zombies. The heroes have to not only fight the villains but also deal with the zombie horde. Another could be an apocalyptic event that wipes out most of the world's quirks, and the remaining heroes and villains must adapt to this new powerless world.
Maybe in the 'bnha graduation fanfic', there's a competition among the students as part of the graduation. The winner gets a special internship or a chance to work with a top hero. This could lead to intense rivalries and unexpected alliances.
Perhaps the OC is a scientist who has been researching a new form of energy. This energy is actually what powers the Sekirei. The OC's research attracts the attention of a group of Sekirei. One Sekirei in particular befriends the OC to learn more about this energy. As they spend time together, the OC begins to understand the Sekirei's world better and helps them find a way to break free from the constraints of the Sekirei Plan.
One idea could be a story where the OC is a traveler who meets Obara in a strange land. They start off as strangers but gradually form a deep bond as they face various challenges together, like surviving in a harsh wilderness or solving the mystery of an ancient ruin.
One idea could be an OC who is a foreign ninja from a far - off land coming to Konoha. Their unique fighting style and culture shock could create interesting storylines. For example, they might introduce new jutsu techniques that the Konoha ninjas have never seen before.
One idea could be a story where Gai and the OC meet during a difficult mission. The OC is initially skeptical of Gai's flamboyant style but gradually comes to respect his skills and positive attitude. They then form an unlikely partnership and face various challenges together, growing closer as they overcome each other's weaknesses.
One idea could be a story where Legoshi and the OC meet in an unexpected place, like a hidden part of the school. The OC could be a new student who is a bit of an outcast, and Legoshi is drawn to their unique personality. They start to bond over their shared love for a particular type of music or art.