Sirius could also have left some personal mementos to his other friends in the wizarding world. For example, he might have left some old photos or letters to Remus Lupin that held memories of their time at Hogwarts together. These could be things that were precious to them both and a way for Sirius to leave a part of himself with his friend even after his death.
Maybe Sirius left his possessions to Harry. His beloved motorcycle could be bequeathed to Harry, as it was something very special to him and also had a connection to Harry's escape from the Dursleys in the past.
Harry might be initially shocked. He could feel a mix of sadness and a sense of responsibility. After all, Sirius was like a father figure to him.
Well, Harry might start to look at Sirius in a new light. He could see him not just as his godfather, but as a figure that inspired creativity in others. This could lead to Harry wanting to learn more about Sirius's past, things that might not have been covered in the original stories he knew. And it could also lead to him having more conversations with people who wrote or read the fanfiction, getting new perspectives on Sirius.
Sirius would likely be extremely angry. He already has a protective streak towards Harry. He might immediately confront those he suspects of the abuse, whether it's the Dursleys or others at Hogwarts. He would probably also try to remove Harry from the situation as quickly as possible.
One possible reason could be a misunderstanding. Maybe some false information was spread about Harry that made Sirius believe he had done something unforgivable. For example, if there were rumors that Harry was involved with dark magic in some way, Sirius, who has a strong aversion to the dark side, might be quick to disown him.
Sirius might use magic to punish them. He could hex them or use some other form of magic to make them feel the pain they inflicted on Harry.
Sirius could face serious trouble. Snape would surely report him to Dumbledore. Dumbledore might have to take action like restricting Sirius' access to Hogwarts or even sending him back to Azkaban if the harm to Harry was severe enough.
Harry would likely be quite curious. He'd want to know what these fanfictions were about. Maybe he'd be a bit cautious at the same time, worried that some of the fanfictions might distort Sirius's true character. But overall, I think he'd be intrigued and might even try to find a way to read some of them, just to see what the fuss was all about.
Well, perhaps Sirius trains Harry in advanced defensive magic. Harry could learn spells like the Patronus charm more effectively under Sirius's tutelage. Sirius might also teach Harry about the wizarding world's secrets that Dumbledore didn't share.
The 'Harry Potter' books are set in a magical world full of its own stories, prophecies, and secrets. Sirius is a complex character with a lot on his plate. He has to deal with his family history, his time in Azkaban, and his loyalty to his friends. Fanfiction is a form of creative writing by fans outside the original story. Sirius doesn't have the time or the inclination within the context of the story to be reading fanfiction. His life is too filled with real magical drama and danger.
Well, if Sirius read fanfiction, it could open up a whole new side to his character. Maybe he'd start to question some of the things he thought were absolute in the wizarding world based on the creative ideas in fanfiction. But realistically, he has so much going on in his life - from trying to clear his name to protecting Harry - that it's hard to imagine him actually taking the time to read fanfiction. He's more of a doer in the real magical world than a consumer of fan - created fictional works.