It could be a group of villains working together. For example, the League of Villains might decide to kidnap Deku. They might want to use him as a bargaining chip against the heroes. Or it could be a rogue hero who has turned to the dark side. This hero might think that Deku is getting too much attention and kidnaps him to take his place in the spotlight.
Well, in a Deku kidnapped fanfiction, perhaps an anti - hero group kidnaps him. They might see Deku as a symbol of the hero system they oppose. They could hold him hostage to make demands on the hero society, like changes in how heroes are regulated. Or it could be a rival from his school days who is jealous of Deku's success and kidnaps him out of envy.
Toga could be an ally. Since she has her own complex relationship with the idea of heroes and villains, she might find Deku's transformation interesting. She could provide some of her unique skills, like shape - shifting, to help Deku in his villainous plans. Additionally, Twice might also join them because of his loyalty to Toga and his own sense of being an outcast in the hero world.
It could be the scientists or villains who turned Deku into a Noumu. They might want to use him for their evil plans, like taking over the hero society.
A possible main antagonist could be a scientist - turned - villain. This character could have created the situation where Deku needs oxygen in the first place. He might be obsessed with creating the 'perfect' hero and when Deku doesn't fit his mold, he decides to try and destroy him. The villain could use his scientific knowledge to create various obstacles for Deku related to his oxygen - dependent state.
In a 'My Hero Academia Noumu Deku' fanfiction, the plot could involve Deku being turned into a Noumu - like creature against his will. This transformation could lead to him being shunned by society at first. However, he could use his newfound strength, along with his unwavering hero spirit, to prove that he's still a hero at heart. He might team up with some unlikely allies, like villains who also oppose the unethical experiments that created him.
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.
It could be an old enemy of Keith's, someone from his past who has a vendetta against him. Maybe a former rival from his days in the military academy who has now turned to more devious means.
In some creative fanfiction, it could be an alternate version of Spiderman himself from a different dimension. This Spiderman has gone bad and kidnaps a fan from our dimension as part of some grand, dark plan that involves crossing dimensions and causing chaos.
A power - hungry nobleman from a neighboring land could be the kidnapper. He might believe that by having Rapunzel in his possession, he can gain some sort of political advantage or use her as a bargaining chip against her kingdom.
It could be Orochimaru. He's always been interested in obtaining powerful individuals for his experiments, and Naruko with her unique abilities would be a prime target.