Obviously, Harry Potter and Regulus Black are the main ones. Harry as the boy who lived, and Regulus as his guardian, would be central to the story. Their relationship would be explored deeply.
A third plotline could be how Harry's Hogwarts experience is different. He might have different knowledge and skills compared to the original story. For instance, he could be more proficient in certain areas of magic that Regulus specialized in, like ancient runes or potion - making. This would give him an edge in his classes and adventures at Hogwarts.
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.
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.
A possible plotline is the impact on Harry's personality. Being raised by Regulus, who was once a Death Eater but had a change of heart, might make Harry more complex. He could have a better understanding of the gray areas in the wizarding world. For example, he might not be as quick to judge Slytherins. And in terms of the overarching story, there could be a new take on the prophecy. Maybe with Regulus' influence, Harry interprets the prophecy in a different way that changes the whole course of the battle against Voldemort.
It's popular because it combines two very interesting characters. Harry is the main hero of the series, and Regulus is a mysterious figure with a complex backstory. Writers can explore what could have happened if they had interacted more directly.
One common theme is redemption. Regulus is often depicted as seeking redemption for his past mistakes, and Harry can be a catalyst for that. Another theme is family secrets. Since both are part of complex wizarding families, there are always secrets to uncover.
Yes, there are likely some fanfictions out there with this concept. Fanfiction often takes creative liberties with established universes, and making Regulus Black Harry's father is an interesting and non - canonical twist. Writers might explore how this would change Harry's story, his relationships, and the whole wizarding world's dynamic.
One common theme is redemption. Since Regulus was once part of the Death Eaters and then turned against Voldemort, many fanfictions focus on his journey to make up for his past mistakes. Another theme is family secrets. The Black family has a lot of hidden history, and fanfictions often explore how Regulus was affected by these secrets and how he dealt with them in relation to Harry Potter's story.
One interesting fanfiction could be about Regulus Black's secret adventures during his time at Hogwarts that were not mentioned in the original books. It might explore his relationships with other Slytherins in more depth, and how he really felt about the Dark Lord's rising power.