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.
One interesting plot idea could be Harry growing up with Sirius's wild and rebellious spirit. He might be more daring in his adventures at Hogwarts, perhaps even forming a secret group earlier to oppose the Dark Arts in a more proactive way. Another could be the exploration of how Sirius's family history and his status as a Black influence Harry's view of pure - blood ideology. He could struggle with the expectations that come with being a Black heir while also trying to uphold the values he learned from his friends and Dumbledore.
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.
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.
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.
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 interesting 'Harry Potter and Regulus Black' fanfiction could be one where Regulus helps Harry in secret ways during his time at Hogwarts. For example, he might leave behind hidden clues about Voldemort's Horcruxes that only Harry can find.
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.