Well, you could write a fanfic where the zombie apocalypse hits during the UA sports festival. The chaos that ensues as the students, teachers, and spectators have to fight off zombies while also trying to protect each other would be intense. Maybe some characters form unexpected alliances in the face of the apocalypse. Or it could be that certain quirks are more effective against the zombies, like Todoroki's ice quirk which could freeze them in place, and the story could revolve around how the characters with different quirks contribute to the fight against the undead.
One idea could be that the heroes have to use their quirks in new and creative ways to fight off the zombies. For example, Bakugo's explosion quirk could be used to blow up large groups of zombies at once. Another idea is that some of the students get bitten and start to turn, creating a moral dilemma for their classmates on whether to save them or put them down. And a third idea could be that the zombie virus is actually a new type of quirk that has gone out of control, so they need to find the source of this 'quirk' to stop the apocalypse.
Deku would probably be very scared at first but then quickly gather his courage and start using his quirk to fight the zombies. He'd also be looking for ways to save as many people as possible. Bakugo would be more aggressive, charging into groups of zombies without hesitation. He might see it as a chance to prove his strength even more. Ochaco would use her zero - gravity quirk to lift zombies and then drop them from a height, trying to take out multiple at once.
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.
One idea could be a story where Fem Todoroki has to overcome her self - doubts while training in a new and challenging environment. Maybe she's sent on a special mission with a group of heroes she doesn't know well, and she has to prove herself.
A fanfic could be set during a school festival. Momo is in charge of one of the booths and Todoroki volunteers to help her. They have to deal with various problems like running out of supplies. Momo creates new items with her quirk while Todoroki helps with keeping things cool (literally with his ice) or heating up things when needed. In the process, they share some personal stories and get closer, realizing they have more in common than they thought.
Some fanfictions explore Dabi's family relationships in unique ways. For example, there are stories that focus on his relationship with his father, Endeavor, before he became Dabi. They might imagine how things could have been different if there had been more communication or understanding between them.
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.