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.
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 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.
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.
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.
In some 'bnha villain deku fanfic', Deku could be turned into a villain by a powerful villain who manipulates his sense of justice. For example, the villain convinces Deku that the current hero system is corrupt and only by destroying it can true justice be achieved. This could lead to some really intense battles between Deku and his former friends.
Another interesting concept is a chatroom that exists secretly among the villains. They plot their next moves, argue about the best ways to take on the heroes, and maybe even have some internal power struggles. It would be exciting to see their different personalities clash in this virtual space.
One idea could be a crossover with Harry Potter. Imagine Deku attending Hogwarts and learning magic while also trying to control his One For All quirk. He could make friends with Harry and they could share their experiences of being the 'chosen ones'.