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.
One common theme is the exploration of Stain's sense of justice. Since he has a very distinct view on what makes a true hero, fanfictions often delve into this concept, comparing it with the more mainstream ideas of heroism in the 'boku no hero academia' universe.
Next, consider the plot. You can base it on a conflict that arises from the two different characters' beliefs or goals. For example, if you cross Stain with a more traditional hero type, they might have a big argument about what it really means to be a hero. You also need to make sure you stay true to Stain's original character while also making the crossover work.
One popular type might be a Stain crossover with other well - known anime characters. For example, a crossover with All Might could explore an interesting dynamic where their opposing views on heroism clash. Another could be a Stain crossover with Deku, perhaps showing how Deku's ideology might be challenged or changed by interacting with Stain.
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.
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.
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.