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.
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.
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.
One idea could be exploring Tomura's past in more detail. Maybe show his childhood before he became a villain, how he was influenced by All For One. Another could be a story where Tomura tries to redeem himself, which would be a huge twist from his normal villain role.
Perhaps Izuku's father is a scientist who was working on a top - secret project related to quirks. He might have accidentally given Izuku his quirk through an experiment. This could lead to a story where Izuku discovers the truth about his quirk and his father's role in it, while also dealing with the ethical implications of such an origin for his powers.
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.
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.
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.
Another plot could be reincarnating as a student in a different class than the main characters. The protagonist then has to work hard to prove themselves and make their own mark in the hero school, while also interacting with the well - known characters in new and unexpected ways. For example, they might form an alliance with a character who was originally an antagonist in the series.
One idea could be a fusion between Deku and All Might. Imagine Deku having not just One For All, but also All Might's combat experience and charisma from the start. It would change the dynamic of his battles and his relationships with other characters. Another idea is fusing Todoroki with a water - based hero, which could enhance his ice and water abilities even more and lead to new and creative ways of using his quirk in combat and rescue situations.