There could be a theme of self - discovery for Harry. In a fic where Sirius 'sucks', Harry has to re - evaluate his own beliefs and values. He might have to learn to rely on himself more and not put so much faith in others. Additionally, the theme of family dysfunction could be present, as Sirius is part of Harry's family in a way, and a 'bad' Sirius can represent a broken family dynamic that Harry has to deal with.
A possible theme is misjudgment. The fanfiction might explore how Harry misjudged Sirius Black as a great and reliable figure, only to find out that he has some major flaws. Also, the theme of broken trust can be there. If Sirius is not as good as he seems, it can lead to a breakdown of the trust Harry has in him, which can then be a central part of the story's plot and emotional turmoil.
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.
One common theme is mentorship turned into something more. Sirius often starts as a mentor - like figure to Harry, and then the relationship evolves. Another theme is the exploration of family - like bonds that transform into a deeper connection.
One common theme is the exploration of a different kind of friendship. Since Harry is female, the dynamic between her and Sirius is often re - imagined. Another theme is empowerment. The female Harry usually has to face challenges in a male - dominated wizarding world, and Sirius is there to support her growth.
One common theme is loyalty. Sirius Black is known for his loyalty in the original series, and in slash fanfiction, this loyalty often extends to his relationship with the other character. Another theme is the struggle against the prejudices of the wizarding world. Since slash relationships might not be as accepted in that world, they have to fight against that.
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 the idea of forbidden love. Since Sirius is an older figure in Harry's life, it's like a Romeo - Juliet type situation where society might not approve. Another theme is the power of connection and how their shared experiences at Hogwarts and in the fight against Voldemort can strengthen their bond romantically.
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.
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.
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.
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.