They could talk about Harry's parents. Sirius was Harry's father's best friend, so he could share many memories of James and Lily, like how they met at Hogwarts, their adventures in the Marauders' era, and what they were like as young wizards.
Maybe Harry noticed Ginny acting strangely in the fanfiction and he wants Sirius' advice. Sirius has a lot of life experience and wisdom, so Harry might think he can help figure out what's going on with Ginny.
Their talk could also enhance Harry's magical knowledge. Sirius could teach him spells or magical techniques that he hadn't learned at Hogwarts yet. This new knowledge would be very useful for Harry in his fight against the Dark Arts. Moreover, learning from Sirius would make Harry more independent in his magical growth as he gets advice from someone outside of the school environment.
One reason could be Harry's impulsiveness. Harry often jumps into dangerous situations without thinking, and Sirius, being his godfather and having a protective nature, would shout at him to make him realize the risks. For instance, if Harry decides to go after a Death Eater alone.
It could be Dumbledore. He's always been like a mentor to Harry, so it would make sense for him to be the one to give an important talk, whether it's about Harry's destiny or how to handle the challenges in the wizarding world.
Yes, there might be such fanfictions out there. Fanfiction writers often come up with creative and sometimes out - of - the - box ideas. They might explore the idea of Harry having a different relationship to Sirius in an alternate universe where he could be seen as Sirius's daughter. However, it's not a common or traditional take on the Harry Potter universe.
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.
Sirius could help Harry cope by sharing his own experiences of being an outcast in his family. He could tell Harry that just because his parents did this horrible thing, it doesn't mean he's unlovable. Sirius could also introduce Harry to his friends in the wizarding world, like Remus, who could also offer support and friendship.
It's possible that in the cave, they start to see each other as equals rather than just a mentor - student relationship. They might argue about the best way to deal with the cave's mysteries at first, but through that process, they learn to value each other's perspectives. Also, if one of them gets injured in the cave, the care and concern shown by the other would enhance their relationship.
Lupin could be a likely candidate. Given his experience and his role as a sort of mentor figure to Harry, he could give Harry the talk. It might involve Lupin sharing his own experiences with being a werewolf and how to deal with secrets and prejudices in the wizarding world.