The themes often revolve around protection. Sirius was always protective of Harry, so that's a big part of these fan fictions. Also, the theme of self - discovery for Harry, as Sirius helps him understand his identity as a wizard and as a member of the Potter family. Additionally, friendship is a theme as their relationship is a special kind of friendship that goes beyond normal wizard - wizard relationships.
In Harry Sirius fan fiction, a major theme is redemption. Sirius had a troubled past, and these stories often show how he tries to make up for it through his relationship with Harry. Another common theme is the exploration of the wizarding world from their combined perspectives. They might go on adventures together, and through that, we see different aspects of the magical world that we might not see in the original books.
One common theme is loyalty. Their friendship was always based on loyalty in the original stories, and in fan fictions, this loyalty often translates into a deep and abiding love. For example, Sirius is always loyal to Remus even when others might shun him because of his werewolf condition.
One common theme is mentorship. Sirius often acts as a mentor to Harry, guiding him in the wizarding world. Another is family. Since Sirius is like family to Harry, the fics explore the concept of a non - traditional family. And there's also the theme of adventure as they usually go on some sort of adventure together.
Another theme is family. Since Harry lacks a proper family, Sirius becomes like family to him. Fanfictions explore how they create a family - like bond in the absence of Harry's real family. It can be really emotional.
One common theme is loyalty. Sirius is extremely loyal to Harry, and vice versa. In many fanfictions, this loyalty is what gets them through tough situations, like when Harry is facing danger from Voldemort.
Rebellion against the status quo can also be a theme. Sirius is a bit of a rebel, and he encourages Harry to question things. So in many fanfics, they might be involved in actions that go against the traditional rules of the wizarding society. And of course, the theme of friendship - a deep, unbreakable friendship that could be seen as more than just friendship in some interpretations.
Adventure is also a big theme. Just like Harry had his own adventures at Hogwarts, fanfictions often have James Sirius getting into various escapades and Harry either being involved directly or guiding him from the sidelines. It gives a sense of passing on the torch of bravery.
Another theme is family. Since Harry lost his parents, Sirius becomes like a surrogate father figure. Fanfics explore how Sirius fills that void in Harry's life, like in some where they celebrate Christmas together in a very family - like way. It gives Harry the sense of a real family that he's been longing for.
Well, the themes often include redemption. Sirius has a complex past and in many fanfictions, he gets a chance to fully redeem himself. Also, the theme of mentorship is common. Sirius guiding Harry through the difficulties of the wizarding world. And there's the theme of friendship. Their bond that goes beyond the normal wizard - wizard relationship. In these fanfictions, they often rely on each other in tough situations which shows the depth of their friendship.
The theme of loyalty is quite prevalent. They are loyal to each other no matter what. Fanfics might show how this loyalty is tested and how they always come out on top. Also, the exploration of the magical bond between them, which could be stronger than just a normal friendship or family tie, is a common theme.
One common theme is the idea of second chances. With Harry deaged, Sirius gets a chance to be a better guardian or mentor, sort of like a redo of their relationship. For example, in many stories, Sirius is able to prevent some of the traumas Harry faced in the original story.