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.
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.
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 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.
Friendship is a very common theme. Their bond is often depicted as something extremely strong and unwavering. They are shown to be there for each other no matter what.
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 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.
One common theme is redemption. Sirius' father may have been a strict or even cruel figure in the original story, but in fanfiction, he could be seeking redemption for his past actions towards Sirius. Another theme is legacy. How does the Black family legacy affect the relationship between father and son? It could be a burden or a source of pride.
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.
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 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.