A good plot could be a villain being sent to an alternate dimension where they experience a life without quirks. They see how normal people live and cooperate, and this makes them realize that their use of power for evil was wrong. When they return to their own world, they are determined to reform.
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.
Dabi would also work well. His vendetta against Endeavor could be a starting point for his reform. As he discovers more about his family and the truth behind his father's actions, he might decide to change his ways and use his power for good instead of revenge.
One idea could be a hero with an illusion quirk who uses it to create false distractions during battles. For example, making the villains see an army of heroes when there are really only a few, causing them to waste their attacks and energy.
One idea could be exploring how the characters' relationships change. For example, if Deku was a girl, how would her dynamic with Bakugo be different? Maybe Bakugo would be more protective in a different way. Another plot could focus on how the female versions of the male heroes deal with the challenges in the hero society, like discrimination that might be specific to female heroes. Also, a plot could revolve around a genderbend All Might and how she inspires the new generation of heroes.
One idea could be focusing on the children of the main BNHA characters dealing with new villains who have unique powers. They could struggle to live up to their parents' legacies while also finding their own identities.
One idea could be to create a villain OC who has the power to manipulate time in small increments. This could lead to really interesting scenarios in battles against the heroes. For example, they could slow down time just enough to dodge attacks and then speed it up suddenly to surprise their opponents.
Maybe the ghosts are the victims of villains. They seek Izuku's help to bring their killers to justice. This could add a whole new layer of moral and emotional complexity to the story. Izuku would have to balance his regular hero duties with this new mission of helping the ghosts find peace.
A plot could revolve around a new transfer student who brings the game to UA. As the students play, it causes some friendly rivalries and new friendships to form. For instance, when a card reads 'The most embarrassing hero costume flaw', the students' answers could lead to some good - natured teasing and bonding. This could also be a way to explore different characters' sense of humor.
One idea could be that the stalker is a villain who wants to expose All Might's secrets to undermine his heroic image. Another could be a misguided fan who thinks they are protecting All Might but their actions are actually causing trouble. Maybe the stalker has some sort of connection to All Might's past that they are trying to uncover.