Love and Romance are very common. Since it's about Sirius Black and an OC, the development of their relationship, from the first meeting to falling in love, is often a central theme.
Another common theme is the OC's integration into the wizarding world. The OC usually has to navigate the complex world of Hogwarts and the wizarding community, often with Sirius as a guide or a partner in this journey. For example, in many fics, the OC has to learn about magic, magical creatures, and the social hierarchy, all while dealing with her feelings for Sirius.
One common theme is loyalty. Sirius is extremely loyal to Harry, and this is often explored in fanfictions. Another theme is family. Since Sirius is like a father figure to Harry, stories often revolve around the idea of building a family. And the theme of redemption also appears, as Sirius has a complex past that he tries to make up for in relation to Harry.
One common theme is forbidden love. Since Sirius has his own complex situation within the wizarding world, often the relationship with the OC is something that is not easily accepted. For example, in many stories, his family might oppose it.
A theme that frequently shows up is the exploration of identity. With female Harry Potter being a different take on the original character, she is often on a journey of self - discovery, and Sirius plays a role in that. Also, the theme of family is important. Sirius can be seen as a sort of surrogate family member, and the fanfiction might explore how that relationship changes into a romantic one.
The theme of defying expectations also comes up a lot. The relationship between them goes against what is considered normal in the wizarding world. Fanfictions play with this idea, showing how they deal with the judgment and obstacles that come their way because of their relationship. It's about them finding their own way in a world that may not understand their bond at first.
Love against all odds is a very popular theme. Given Sirius's status as a wanted man at times and his family's dark reputation, a relationship with the reader is full of obstacles. Also, the theme of loyalty is often present. The reader and Sirius are loyal to each other in the face of all the chaos in the wizarding world. This loyalty is what binds them together in many stories.
One common theme is family. Sirius is Harry's godfather, so many fanfictions explore the idea of family that goes beyond blood. Another theme is redemption. Sirius has a past full of mistakes and he often tries to redeem himself through his relationship with Harry. Also, protection is a big theme. Sirius is always trying to protect Harry from the threats in the wizarding world.
The theme of redemption is also quite prevalent. Sirius has a complex past, and fanfictions often deal with him trying to make up for his mistakes or be redeemed in the eyes of others. And of course, the power of friendship is a big theme, as their relationship is rooted in that as well.
The theme of second chances is also prevalent. In time travel stories, both Harry and Sirius may get opportunities they missed in the original timeline. For example, Sirius could save his friends who died earlier. And also, the exploration of the past and how it shapes the future is a key theme. They go back and learn things that change their perspective on the present and future of the wizarding world.
Love and friendship are also big themes. The OC and Sirius usually start as friends or strangers and then develop a deep romantic relationship. Their love story might be set against the backdrop of the wizarding world's turmoil, like the fight against Voldemort. It adds an element of danger and excitement to their relationship.
One common theme is friendship. They were very close friends in the original story, and fanfics often explore the depth of their friendship, how they support each other through various situations.