His character could develop in a darker direction. The Venom symbiote often brings out the worst in its hosts. So Harry might become more aggressive and self - centered. He could start to view his friends as obstacles and become more isolated.
One idea could be that Harry Osborn, in his search for power to overcome his family's legacy of misfortune, somehow comes into contact with the Venom symbiote. Maybe he initially tries to control it for his own gain, like using it to take over Oscorp. Another plot could involve Venom influencing Harry's darker side, causing him to clash with Spider - Man in new and unexpected ways.
Yes, there are many 'Harry Osborn' fanfics. You can find them on various fanfic websites like Archive of Our Own. There are different genres and themes related to his character, from friendship stories within the Spider - Man universe to his own adventures.
One good theme is his transformation. Fanfics can explore how he changes, like when he becomes the Green Goblin in some storylines. It can be interesting to see the internal struggle and how it affects his relationships.
A fanfic that stands out is 'Harry and Peter: The Unbreakable Bond'. It focuses on how they support each other. There are many scenes where they have heart - to - heart conversations that give a deep insight into their characters. The author does a great job of staying true to the essence of Peter and Harry in the original stories.