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 common theme is family loyalty. Sirius and Regulus come from the Black family, and fanfictions often explore how their loyalty to family, or lack thereof in Sirius' case, affects their actions and relationships. Another theme is redemption. Regulus, in particular, is often given a chance at redemption in these stories as he had a more complex and morally ambiguous role in the original series.
I really liked 'Regulus' Redemption in the Potter World'. This fanfiction focuses on Regulus' character. It has some really interesting interactions between him and Sirius. It imagines a scenario where Regulus tries to make amends for his past actions and how Sirius reacts to this new - found change in his brother. It also ties in with Harry's story in a very creative way.
The 'sirius hurts regulus fanfiction' often delves into the family aspect by highlighting the expectations placed on them as Blacks. Sirius's actions, like running away from home, can be seen as a betrayal in Regulus's eyes. This causes a rift between them. The fanfictions explore how family loyalty and the need for acceptance within the family are intertwined with the pain that Sirius's choices inflict on Regulus.
At first, it might be a bit of a rocky start. Harry, having grown up without a proper family, could be wary of Regulus. But as time passes, he starts to see Regulus as a father figure. Regulus, on the other hand, is determined to make up for his past mistakes by raising Harry right. So he showers Harry with care and magical knowledge, and they gradually form a strong bond.
The 'Harry Potter' books are set in a magical world full of its own stories, prophecies, and secrets. Sirius is a complex character with a lot on his plate. He has to deal with his family history, his time in Azkaban, and his loyalty to his friends. Fanfiction is a form of creative writing by fans outside the original story. Sirius doesn't have the time or the inclination within the context of the story to be reading fanfiction. His life is too filled with real magical drama and danger.
One common theme is redemption. Regulus was once a Death Eater, and in many fanfictions, he tries to atone for his past sins through his connection with Harry. For example, in stories where he helps Harry uncover secrets about Voldemort.
One interesting plotline could be a mystery - solving one. Harry discovers some strange artifacts in Grimmauld Place that were left by Regulus. With Sirius (if he's alive in the fic), they try to figure out what these artifacts mean and it leads them to uncover a long - forgotten plot by Voldemort. Another plotline could be a redemption arc for Regulus. Harry, after learning about Regulus's attempts to undermine Voldemort, tries to clear his name in the wizarding world, which causes some friction with those who still see the Black family as mostly dark wizards. And there could be a plotline about family secrets. Sirius and Harry find out that there are hidden magical abilities in the Black family bloodline that only Regulus knew about, and they have to figure out how to use or control them.