Well, Sirius would surely focus on making Harry as self - sufficient as possible given his blindness. He might train Harry to use his wand in a more instinctive way, relying on muscle memory and his other senses. Maybe Harry would learn to sense the presence of dark wizards through vibrations in the air or changes in temperature that normal wizards might not notice.
One possible aspect of this fanfiction could be how Sirius's unique personality shapes Harry's upbringing. Sirius is known for his rebellious and loyal nature. His influence might lead Harry to be more daring and less afraid of breaking the rules when it comes to fighting against the Dark Arts. For example, he could teach Harry some unorthodox defensive magic techniques that are not taught at Hogwarts.
In this kind of fanfiction, Harry would likely grow up with a more hands - on and perhaps wilder form of parenting. Sirius was brave and impulsive. He might teach Harry some of his more unorthodox magic skills earlier. Harry would also be surrounded by Sirius' loyalty to his friends and family, which could shape Harry's own values in a different way compared to growing up with the Dursleys.
One possible storyline could be that Bellatrix, despite her dark past, has a change of heart due to some magical influence or a deep - seated love for the Potter family. Sirius, of course, would be a positive influence. They might raise Harry in a more unorthodox way, teaching him both light and dark magic in a balanced way so he can understand the full spectrum. This could lead to Harry being a more powerful and empathetic wizard, able to bridge the gap between different magical factions.
It could lead to a very different dynamic. Harry might have grown up with more freedom and a stronger sense of family.
In a 'Harry and Sirius join Voldemort' fanfiction, it might start with Voldemort making a very tempting offer. For Harry, it could be something related to saving his friends or family. Sirius might be lured by the idea of protecting Harry in a different way. However, once they join, they would face a moral dilemma. They would have to pretend to be on Voldemort's side while still holding onto their own values. And there would be a lot of internal conflict as they see the evil deeds happening around them.
One possibility is that Sirius would teach Harry all about his family's history in a more private and laid - back setting in France. He might introduce Harry to different magical cultures unique to France, like the Beauxbatons Academy. Harry could grow up with a more carefree attitude, away from the prying eyes of the British wizarding world.
Harry could also team up with Hermione and Ron. Hermione would use her extensive research skills to dig into the Ministry's files on Sirius. They might find that there were some irregularities in the trial process. For example, crucial evidence was withheld or fabricated. Ron, on the other hand, could use his family's connections within the wizarding world to gather more information about the people who were involved in the case against Sirius.
Well, Sirius might see some of his own qualities in Harry's boyfriend. Maybe the boyfriend is brave like Sirius was, and Sirius would then start to warm up to him. He could share some of his old Hogwarts stories with the boyfriend, which would help the boyfriend understand Harry's world better.
In fanfiction, when Harry and Ginny meet Sirius, Sirius might offer them some sage advice on dealing with the dark forces. He could share his experiences about fighting Voldemort and his Death Eaters. Maybe he would see the potential in Ginny and encourage her to further develop her magical abilities, as he has always been supportive of Harry.
Often, these fanfictions highlight a strong bond between Sirius and female Harry. The stories might focus on how Sirius imparts his own rebellious and protective nature onto her. For example, he could teach her how to stand up to authority in the wizarding world, like the Ministry of Magic.