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.
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.
Another place to check could be FanFiction.net. It has a large collection of fan - made stories for various series, including My Hero Academia. However, the quality can vary, so you might need to sift through a bit to find the really good Stain fanfictions.
Todoroki might be another likely character. He has a complex power set and has been through a lot of emotional and physical growth. His struggle to control his two quirks could be a parallel to the control needed for Ultra Instinct. If he can master the balance within himself, it could potentially lead to the awakening of Ultra Instinct in him.
The Ultra Instinct in a fanfiction might also have an impact on the character's growth. It could be a new milestone in their hero journey. Instead of just relying on their existing quirks, they have this new ability to master. This could lead to new relationships and rivalries. Other characters might be in awe or jealous of this new power, which would create more drama and tension within the story.
One idea could be exploring Stain's backstory in more detail. Maybe his early life experiences that led him to his extreme views. Another could be a 'what if' scenario where Stain teams up with a hero, albeit reluctantly, to take on a greater evil. And a third idea could be to write from the perspective of someone inspired by Stain's ideology but trying to find a more peaceful way to bring about change in the hero society.