Fanfictions related to Stain from 'boku no hero academia' can vary greatly. Some might depict him as a more sympathetic character, trying to understand his motives for his extreme actions. Others could be more action - oriented, with Stain engaging in battles that we didn't see in the anime or manga. There's also the possibility of exploring his backstory further, like his early days and what led him to become the character we know.
In the fanfiction, perhaps Stain trains Izuku because he believes Izuku has the purity of heart to use power in a way that Stain deems correct. Stain could teach Izuku how to analyze opponents better, which would enhance Izuku's ability to strategize in battles. Izuku might then start to question the current hero ranking system during his training. He could grow to be a hero who is more focused on the essence of heroism rather than just fame and recognition, which would make him stand out among his peers and also cause some friction with the more traditional heroes and hero agencies.
One popular theme is exploring Stain's ideology further. Many fanfictions dig deeper into his beliefs about true heroes and villains, and how that might play out in different scenarios.
There's also 'Growth Together'. This fanfic shows how Deku and Todoroki support each other during tough times at UA. They face challenges not only from villains but also from within the school. Their relationship develops as they help each other overcome these obstacles.
One idea for a Deku's Quirk fanfiction could be exploring what would happen if Deku's quirk manifested in a different way. For example, instead of having to gradually control One For All, he suddenly has full mastery from the start. This could lead to a very different story arc where he is immediately seen as a top - tier hero - in - training, causing envy and admiration in equal measure among his classmates.
One idea could be a fanfiction where Deku's quirk suddenly mutates. Instead of just having One For All, he starts to develop an additional power related to it, like the ability to share a fraction of One For All's power with his friends temporarily to boost their own quirks in a time of great need.