Well, it could be a doll that has the power to control the minds of the characters. Imagine a doll that makes the heroes act against their will, like making Deku use his power to harm his friends instead of save them. That would be really terrifying.
One terrifying plot could be a doll that has the ability to create illusions. It makes the characters see their worst nightmares as if they were real. For example, Bakugo might see himself as a villain who has successfully destroyed all his friends and the city. The doll could be using these illusions to feed on the negative emotions of the characters, making them weaker and more vulnerable to its real attacks.
Maybe a doll that seems innocent at first but is actually a vessel for an ancient evil. It slowly corrupts the world of My Hero Academia, starting with small things like making plants die around it. As it grows in power, it starts to possess the students, one by one, and they begin to act in very strange and menacing ways. It could be hidden in plain sight, perhaps in a forgotten corner of the school, and no one suspects it until it's too late.
Perhaps an unknown entity that has possessed the doll. This entity could come from another dimension where evil rules. It uses the doll as a form to enter the My Hero Academia world and plans to take over. It starts by causing strange and unexplainable events, and as the heroes try to figure out what's going on, it gets stronger and more menacing.
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.
Overall, the My Hero Academia manga doll is of satisfactory quality. The materials used are nice, and the design closely resembles the characters from the manga. However, some minor flaws might be present in the finishing touches.
To make your fanfiction stand out, focus on unique character developments. For example, take a minor character and give them a major role in your creepy story. Maybe make Mineta the center of a really disturbing plot twist. Also, use vivid descriptions. Instead of just saying 'it was a dark room', say 'the room was a void of inky blackness, the walls seemed to ooze a cold, clammy substance that made the air feel heavy and suffocating'. And don't be afraid to play with the existing lore in new and unexpected ways.
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 that the Saiyan OC is mistaken for a new type of villain at first. As they try to prove they're not a threat, they start to make friends and learn about heroism. Another idea is that they discover a connection between Saiyan powers and some of the hidden secrets in the MHA world. Maybe there's an ancient power source that only a Saiyan can unlock.
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 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.
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 possible plot could be Deku facing a villain who has the ability to manipulate air or oxygen levels. Deku has to use his quick thinking and newfound control over One For All in a low - oxygen environment. Maybe he discovers a new aspect of his power that allows him to function better on less oxygen.