Fanfiction about these three can take many forms. For example, there could be time - travel fics where Harry goes back in time and meets Regulus and Sirius when they were younger. This could lead to all sorts of interesting plot twists, like Harry trying to prevent Regulus from joining the Death Eaters in the first place. Or it could be a fic where Sirius survives and he and Harry explore the secrets of the Black family together, including things that Regulus left behind. There are also fics that focus on the emotional bonds between the characters, showing how the different personalities of Regulus, Sirius, and Harry interact and influence each other.
One great Sirius Black x Reader fanfiction is 'The Black Dog's Mate'. It beautifully captures the essence of the relationship with Sirius. Another is 'Sirius and the Unlikely Bond' which has a really interesting plot where the reader character is from a different magical family. 'A Sirius Love' is also a good read, with lots of emotional moments between the two characters.
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.
There's 'The Unforeseen Bond'. In this fanfiction, the fem Harry Potter's story unfolds in a way that she and Sirius Black are drawn to each other in a rather unexpected manner. It delves into their shared experiences and how they support each other through various challenges.
One great 'Harry Potter x Sirius Black fanfic' is 'The Marauders' Reunion'. It beautifully captures their relationship and the magic of the wizarding world. Another is 'Sirius' Redemption' which delves deep into Sirius' character in relation to Harry. And 'Bond Beyond Blood' shows a really interesting take on their bond that goes beyond the typical mentor - mentee relationship.
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.
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 possible reason could be a misunderstanding. Maybe some false information was spread about Harry that made Sirius believe he had done something unforgivable. For example, if there were rumors that Harry was involved with dark magic in some way, Sirius, who has a strong aversion to the dark side, might be quick to disown him.