In 'hero academia sludge fanfiction', you might consider a story where the sludge is actually a misunderstood entity. It was once a normal substance but got corrupted by some evil force. A hero could try to purify the sludge instead of just fighting it. They might have to go on a quest to find the source of the corruption and in the process, learn more about the true nature of the sludge.
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.
Maybe the sludge villain has a sidekick or an accomplice who is enraged by his death. This person decides to take revenge on the heroes, especially the ones directly involved in the sludge villain's demise. They start targeting the hero's friends and family, creating a more personal and dangerous threat for the heroes.
First, do some research on the existing 'hero academia' world. Know the characters, their powers, and the general setting. Then, think about how the sludge fits into this world. You could start with a scene where the sludge makes its first appearance, like it oozes out during a big battle.
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.
A cool concept for My Hero Academia fanfiction is to have a time - travel plot. A character could go back in time and change a significant event in the My Hero Academia world. This could lead to a completely different storyline. Or you could write a story centered around a villain redemption arc. For instance, giving a more in - depth look at how a villain like Shigaraki could potentially be redeemed and how the heroes would react to that. Additionally, fanfiction about a tournament between different hero agencies could be really exciting.
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 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.
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.
One idea could be a story where Eraserhead and Ms. Joke team up on a particularly difficult mission. Maybe they have to infiltrate a villain's lair and their different quirks work in harmony to overcome various obstacles.
One idea could be a story where a character's quirk goes horribly wrong in a very creepy way. For example, if Deku's One For All started to consume his mind instead of empower him, making him lose his sense of self and become a menacing figure. Another could be about a hidden villain within UA who uses their quirk to create disturbing illusions that drive students mad. And a third idea might be a fanfiction where the Nomu are given a more sinister backstory, like they were once normal people whose quirks were brutally experimented on, turning them into the terrifying creatures they are now.