One common theme is her struggle with the Black family legacy. She has to deal with the reputation of her family, both the good and the bad. Another theme is her relationships, like with her father's friends or other Hogwarts students. And often, there's a theme of self - discovery, as she figures out who she is in the wizarding world.
Family relationships are also a big theme. These fanfics usually show how Hermione and Sirius interact with their daughter, and how the family dynamic works. It could be a loving and nurturing environment or one filled with challenges as they try to protect her from the dangers of the wizarding world. Magic and power is another common theme. Their daughter is often depicted as having strong magical abilities, and the fanfics explore how she uses and controls her power.
One common theme is the struggle with identity. Since her father was Sirius Black, she has a complex family history. She often has to figure out who she is in the shadow of her father's fame and infamy. Another theme is loyalty. She might be loyal to her father's friends, like Harry, Hermione, and Ron, and fight alongside them. Also, the theme of magic discovery is common. She discovers new magical abilities that are either related to her father or are completely unique to her.
Another theme is redemption through love. Sirius has had a troubled past, and in many fanfics, the love of a particular character helps him come to terms with his past mistakes and find a new path in life. It gives him a reason to be a better person.
The theme of freedom is quite important too. Sirius was locked up in Azkaban for a long time, and when he escaped, he was fighting for his freedom. Fanfictions often expand on this, showing his journey to fully regain his freedom and his place in the wizarding world, while also dealing with the prejudice against him because of his supposed crimes.
In 'Harry is Sirius' daughter' fanfiction, a prevalent theme is the exploration of alternative family structures. Given that the original Harry Potter series had Harry orphaned and with a rather complex family situation, this new concept completely reimagines his family. Another theme is the impact on existing relationships. For instance, how would Harry's relationships with her friends at Hogwarts change? Would they treat her differently knowing she was Sirius' daughter? There's also the theme of power dynamics. Sirius was a powerful wizard, and if Harry was his daughter, how would that affect her own power growth and development? How would the wizarding community view her in light of this new family connection? And finally, the theme of secrets and revelations is often present, as Harry discovers this new aspect of her identity and has to deal with the consequences of that discovery.
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.
A recurring theme is trust. Since they are part of the Marauders, they trust each other with their lives. In fanfics like 'Remus and Sirius: The Trust Factor', it details how their trust in each other is unwavering. Also, the idea of shared secrets is often explored. They both have secrets (Remus's werewolf form and Sirius's family secrets) and in some fanfics like 'Remus and Sirius: Secrets Shared', it shows how they handle and protect each other's secrets within the context of their friendship.
The theme of redemption also shows up. Since Sirius had a somewhat 'tainted' reputation in the eyes of the Black family, some fanfictions explore if Orion could have played a role in his redemption or if Sirius could have redeemed the family name in Orion's eyes.
One common theme is the vulnerability of Sirius. Usually, he's portrayed as this strong, brave character, but when sick, he shows a different side. Another theme is the support from his friends and family. They often rally around him, highlighting the importance of those relationships.
One common theme is family conflict. Sirius rebels against the pure - blood mania and dark magic - associated ways of the Black family that Walburga represents. Another theme is the struggle for identity. Sirius is trying to be his own person away from the expectations set by his family with Walburga as a symbol of those family constraints.