A plot could revolve around the son being sorted into a different house than expected. For example, if he's sorted into Hufflepuff instead of Slytherin like his father. He then has to deal with the disappointment of some Slytherins who expected him to carry on the family legacy in their house, while also making friends in Hufflepuff who help him see the value in his new house. He could also be on a journey to uncover the truth about his father's redemption, and in the process, he uncovers a plot by some remaining Death Eaters to use his name for their own dark purposes.
One plot idea could be that the son discovers his father's past with Voldemort and tries to make amends for it in the wizarding world. He could use his unique position as the son of a former Death Eater and a hero's friend to bridge gaps between different factions. Another idea is that he has some special magical abilities inherited from both his father and the Potter side, which he has to learn to control while dealing with the expectations of others. And he might go on a quest to find hidden secrets left behind by his father related to the Horcruxes.
The son could be a key figure in a new threat to the wizarding world. Maybe some dark magic related to his father's past is resurfacing, and he is suspected of being involved because of his lineage. However, he's actually trying to stop it. He teams up with some of Harry Potter's friends' children, and together they face this new danger. They explore old wizarding locations that were important to his father, like the cave where the Horcrux was hidden, to find clues on how to defeat this new evil.
A third plot idea is that Harry's connection to Regulus Black affects his relationship with Sirius. Instead of being his godfather, Sirius could be more of an uncle figure. And Harry could be more involved in the Black family affairs. He might learn about the family's dark secrets and try to redeem the family name. This could involve dealing with the Black family's old enemies and allies, and Harry would have to navigate this complex web of relationships while still being the Chosen One.
One idea could be that Harry discovers his true parentage and has to struggle with his identity. He might be torn between the side of good that he has known all his life and the dark legacy of Bellatrix. Another plot could involve him facing discrimination from both the wizarding world that hates Bellatrix and the Death Eaters who might see him as a tool or a threat. For example, he could try to use his knowledge of Bellatrix's magic, which might be more complex and dark - tinged, to fight against Voldemort in a different way than in the original story.
The story would also shift in terms of Harry's place at Hogwarts. His sorting into Gryffindor might be more complex. Since he's related to the Black family, which has a history of being in Slytherin, there could be an internal struggle within Harry about his house loyalty. And his friends, like Ron and Hermione, might react differently to this new revelation about his parentage as they've always known him as a Potter.
Well, in such fanfictions, it could be that the author invents new prophecies or backstories. They might say that there was a mix - up at the time of Harry's birth, and that he was actually Regulus's son all along. Or perhaps there are some items or memories that only make sense if Harry is related to Regulus, like a locket that has a special connection to both of them.
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 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.
One idea could be exploring Regulus Black's time as a professor and his relationships with students. Maybe he has a student who reminds him of his younger self, and he tries to guide them away from the dark path he once walked. Another could be a mystery where he has to solve a magical riddle in the school that is linked to his past secrets.
One plot idea could be that Regulus returns as a ghost and tries to guide Harry Potter in secret. Another could be his return through some dark magic experiment gone wrong but he has new powers and knowledge. Maybe he returns to expose the true nature of certain Death Eaters who are still at large.
Another plot could be that Regulus, reborn, has memories that are fragmented. As he tries to piece them together, he discovers new secrets about the Black family's past that could change the way the wizarding world views his family. Along the way, he forms unexpected alliances with those who were once his enemies.
One idea could be that Regulus Black, despite being a Slytherin in the original story, has a secret admiration for Gryffindor values. So, in the fanfiction, he could start secretly taking part in Gryffindor - like adventures. For example, he might help Gryffindor students in a fight against some dark forces in the Forbidden Forest.