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.
Well, it could be that the author has a very different interpretation of Sirius Black's character. Maybe they focused on some of his flaws or actions that they didn't like and decided to write a fic based on that. Or perhaps they wanted to create a unique storyline where Sirius Black is not the 'hero' he is often portrayed as in the original Harry Potter series.
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.
It could develop in a complex way. The spanking incident might initially create tension between them. Harry could be angry or resentful towards Sirius at first.
Another aspect of this fanfiction could be that Harry uses his Parseltongue ability to communicate with the snakes in Azkaban (if there are any). The snakes might have some information about Sirius since they can sense the emotions and secrets of the prisoners. Through this unique method, Harry gets closer to proving Sirius's innocence.
Perhaps in the fanfiction, Sirius is mad at Harry because Harry has befriended someone Sirius doesn't trust. Sirius, being protective of Harry, might see this new friendship as a threat. He could be worried that this person will lead Harry astray or put him in danger, and his anger towards Harry stems from his concern for Harry's well - being.
Well, a popular one is 'Sirius and Harry: A Different Kind of Family'. In this fanfiction, Sirius actually gets a chance to be a proper father figure to Harry. It shows their relationship evolving in a very heartwarming way. There's also 'The Sirius - Harry Connection'. This one plays around with the idea of a magical bond between them that goes deeper than what was shown in the books. And 'Harry's Refuge with Sirius' where Sirius provides a safe haven for Harry during tough times at Hogwarts.
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.
In this type of fanfiction, the relationship is often explored through emotional depth. For example, Sirius might be portrayed as a mentor to Harry, guiding him through the difficult world of magic. It could also show their shared experiences of loss, which bonds them closer. Maybe Sirius shares his memories of James with Harry, strengthening their connection.
One great 'Harry Potter Sirius Black fanfiction' is 'Padfoot Returns'. It really delves into Sirius's character and his relationships within the wizarding world. The story has a great plot that keeps you hooked from start to finish.