An interesting plot could involve an anti - hero who has a hidden agenda that is not entirely evil but also not in line with the traditional hero goals. Maybe they are trying to expose a corrupt part of the hero society from within. They could form alliances with some villains who also have a vendetta against the corrupt elements. This would create a complex web of relationships and moral dilemmas.
How about a fanfiction where the villains and heroes have to team up? There could be a greater threat, like an alien invasion or a powerful quirk - user who is out to destroy the entire world. The heroes and villains realize that they can't defeat this threat alone, so they put aside their differences. This would lead to some really interesting character interactions, like Bakugo having to work with Shigaraki and both of them learning to respect each other's skills despite their opposing natures.
One plot idea could be a villain who is secretly a double agent, working for the heroes all along. They infiltrate the villain group and gather information. Another idea is a hero who loses their quirk and has to rely on their wits and combat skills alone, like Batman in the DC universe. And a villain - hero team - up to face a greater threat that neither can handle alone.
Maybe a plot where a group of Warhammer heroes are sent to the Hero Academia world to stop a chaos invasion that is starting to seep into that realm. They have to work with the local heroes, sharing their knowledge of dark magic and ancient weapons to combat the new threat. This would involve a lot of cultural clashes and learning experiences between the two groups.
An interesting plot could involve an OC who is initially thought to be quirkless but discovers a latent quirk later on. This OC could then struggle to catch up with the others in terms of training and development. Or, an OC with a shapeshifting quirk could be used in a plot where they have to infiltrate a villain's lair by taking on the form of one of the villains' minions. They would have to be careful not to be discovered and also gather intelligence along the way.
One plot idea could be Naruto being betrayed by the Hidden Leaf Village. He then decides to use his powers for his own gain rather than for the village. He might form his own group of outcasts and fight against the village's corrupt leadership.
Well, you could focus on the relationships between the characters. Write a fanfiction about a friendship that forms between two characters who initially didn't like each other. For instance, Bakugo and Midoriya could have a different kind of relationship where they become more like brothers through a series of events that force them to rely on each other. Or you could write about the teachers' lives outside of teaching at the academy. What do they do in their free time? What kind of challenges do they face that the students don't know about? You could also write a story where a new villain emerges with a quirk that is completely unexpected and throws the whole hero society into chaos.
A fanfiction focusing on the family lives of the characters can be interesting. Like, how does Bakugo's family interact when he's not around? Or what was Midoriya's father really like? By delving into these aspects, you can add more depth to the existing characters and show a different side of them that the original story might not have fully explored.
One idea could be a crossover where the students from Little Witch Academia visit UA High. They could participate in a joint training exercise. The witches could use their magic in creative ways during combat scenarios, while the heroes-in-training show off their quirks. It would be a fun mix of different powers and personalities.
One idea could be a 'what if' scenario where the villains and heroes have to team up against an even greater threat from another dimension. It would be fun to see how characters like Bakugo and Dabi interact in this forced alliance.
One idea could be that during Halloween, the villains use the spooky season as a cover to carry out a major heist. The heroes have to figure out their plan while also dealing with all the Halloween chaos in the city. For example, there could be trick - or - treaters getting in the way and the villains using Halloween decorations to hide their traps.