One interesting plotline could be how Sirius helps Harry discover more about his parents in a more personal way. Since Sirius was James Potter's best friend, he could share countless stories and memories that Harry never knew. Another plotline might involve Sirius teaching Harry some advanced magic that he wouldn't learn at Hogwarts. This could give Harry an edge in dealing with the various threats he faces. Also, their relationship could be explored in depth, showing how Sirius becomes not just a guardian but also a true father figure to Harry.
One common plotline is Sirius Black escaping from Azkaban earlier and secretly training Harry in advanced magic. Another could be Sirius and Harry going on a hidden adventure to uncover more about Harry's parents. There are also stories where Sirius plays a more active role in the fight against Voldemort at Hogwarts, using his knowledge of the Dark Arts and his Animagus form to their advantage.
One interesting plotline could be Harry growing up with the Black family's unique traditions and magic. He might learn different spells and magical knowledge that are specific to the Black family heritage. Another could be his relationship with Sirius Black within the family context, perhaps having a closer bond earlier on.
One interesting plotline could be Harry learning about the Black family's dark secrets while being adopted by them. He might discover hidden magic or family grudges that shape his new life.
One possible plotline could be Harry learning more about his parents from Sirius. Sirius could share all the little details, like how James was in school, his mischievous deeds, and how he courted Lily. This would give Harry a deeper connection to his family's past. Another could be Harry and Sirius training together in magic, especially in defensive spells. Sirius, being a skilled wizard, could teach Harry some unique spells and techniques that he might not learn at Hogwarts. Also, they could have adventures in the wizarding world together, exploring places that are off - limits to most students.
One interesting plotline could be a mystery - solving one. Harry discovers some strange artifacts in Grimmauld Place that were left by Regulus. With Sirius (if he's alive in the fic), they try to figure out what these artifacts mean and it leads them to uncover a long - forgotten plot by Voldemort. Another plotline could be a redemption arc for Regulus. Harry, after learning about Regulus's attempts to undermine Voldemort, tries to clear his name in the wizarding world, which causes some friction with those who still see the Black family as mostly dark wizards. And there could be a plotline about family secrets. Sirius and Harry find out that there are hidden magical abilities in the Black family bloodline that only Regulus knew about, and they have to figure out how to use or control them.
Perhaps a plotline where Sirius Black discovers a hidden prophecy related to Harry Potter that requires Harry to submit to Sirius in a series of magical trials. These trials could be designed to unlock Harry's true potential. Or it could be a plotline where the wizarding world is in chaos, and Sirius, with his knowledge and experience, convinces Harry that submitting to his leadership is the only way to restore order. This could involve Harry following Sirius' orders in battles, in diplomatic relations with other magical creatures, and in reforming the Ministry of Magic.
One interesting plotline could be how Harry adjusts to the Malfoy family's pure - blood traditions while still holding onto his own values. For example, he might struggle with the Malfoys' views on Muggle - borns. Another could be his relationship with Draco Malfoy. Instead of being rivals, they could become brothers in a sense, and their dynamic would change completely, leading to new adventures at Hogwarts together.
One interesting plotline could be that Harry is adopted by a powerful wizard family outside of the wizarding world. They introduce him to different magical cultures and techniques that are not common in Hogwarts. This gives Harry a whole new perspective on magic and makes him even more powerful in the long run.
One interesting plotline could be a character who is adopted into a pure - blood family and has to navigate the complex politics of the wizarding world's blood hierarchy. They might face discrimination from some traditional pure - bloods while also trying to find their place within the family's expectations.
Another possible plotline is the relationship between Harry and the goblin community. How do they accept him as one of their own? There could be some goblins who are against the idea of adopting a human, and Harry has to prove himself worthy. This could lead to a lot of internal conflict within the goblin society as well as Harry's own personal growth.