In fanfiction, Harry Osborn as Black Spiderman might bring a new dynamic to the story. His background as a rich and sometimes troubled character in the original Spider - Man story can add layers. For example, he could use his wealth in ways that a regular Black Spiderman might not. His family history and the influence of his father could play a role in his actions as a superhero. Also, his relationships with villains could be different. Maybe he has a more personal vendetta against some of them because of his family's past. And his relationship with Mary Jane or Gwen Stacy could be completely different from what we expect in the normal Spider - Man story.
One common theme is identity crisis. Harry has to deal with his dual identity as Harry Osborn and the Black Spider. Another theme is redemption. He often has to make up for things he did wrong in the past while being the Black Spider.
There's also 'Black Spider: Harry's Redemption'. In this fanfic, Harry Osborn as the Black Spider is on a journey to redeem himself for past mistakes. It has some really interesting character development and action scenes.
One great fanfiction idea could be Harry Osborn's internal struggle with his new - found powers in 'The Amazing Spider - Man 2'. He might be torn between using them for good, like Spider - Man, or for his own self - interests. For example, he could be constantly fighting the urge to use his powers to gain more control over Oscorp, while also being aware that Spider - Man's way of using powers is more noble.
One idea could be a story where Peter tries to protect Harry from the darker aspects of their world. Since Peter has his Spider - Man powers, he could be constantly on guard to keep Harry safe, perhaps from the enemies he made as Spider - Man. For example, a villain might target Harry to get to Peter.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions, and you can easily search for Peter Parker and Harry Osborn stories using the search bar. Just type in their names and you'll get a list of relevant fanfictions.
There's also 'Friendship Forged in Crisis' which is an amazing Peter Parker and Harry Osborn fanfiction. It focuses on a major crisis in the city where they have to rely on each other completely. The story has a lot of action and also some really touching moments between the two characters that really bring out the essence of their friendship.
A fanfiction could focus on Harry Osborn's life at school. He's rich and popular, but in this version, he's also secretly dealing with family issues related to his father's business. He could cross paths with Peter more often at school, and their interactions could be a mix of rivalry and a budding friendship.