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 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.
One idea could be exploring Overhaul's backstory in more depth. Maybe show his childhood and what led him to become the villain he is. Another could be a redemption arc for him, where he teams up with the heroes against a greater evil.
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.
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.
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.
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 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.
One idea could be a crossover with 'One Piece'. The heroes from 'boku no hero academia' could end up on the Grand Line and have to deal with pirates while also using their quirks. Another is a crossover with 'Naruto'. Maybe the students of UA could train with the ninja in Konoha, learning new skills and sharing their quirk knowledge. A third could be a crossover with 'Attack on Titan', where the heroes have to face the Titans and figure out how to use their quirks to defeat these huge threats.
One interesting idea could be how the characters' relationships change. For example, if Deku was a girl, his interactions with Bakugo might be different, perhaps less aggressive from Bakugo's side. Another idea is the change in character design. A female All Might might have a different style of hero costume, and her way of inspiring others could also be distinct. Also, the storylines could shift, like a female Todoroki dealing with her family issues in a more feminine - yet still strong - way.
In a 'boku no hero academia the gamer fanfiction', the main character could be a transfer student who has a gamer - inspired quirk. They could use this quirk to create strategic plans for hero battles. For instance, they can map out the battlefield in their mind like a game level. They might also be able to recruit other heroes by offering them in - game - like rewards. This would create a whole new dynamic in the relationships between heroes. Additionally, they could use their knowledge of gaming glitches to find weaknesses in villains' defenses.