One common plotline is Regulus joining the fight against Voldemort after faking his death. He uses his knowledge of the Death Eaters to help the Order of the Phoenix. Another is his relationship with Sirius. They get a chance to reconcile and work together.
A really interesting plotline is when Regulus is alive and has to deal with the aftermath of his actions as a Death Eater. He tries to make amends to the families he's hurt. And sometimes, there's a romantic plotline where he falls in love with a character who helps him through this redemption process. It could be a Muggle - born witch or someone from the Order. There are also stories where he discovers secrets about his family that change his perspective on everything.
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.
One idea could be that Regulus, after faking his death, has been secretly working against Voldemort all this time. He could be gathering intelligence and using his knowledge of the Death Eaters to undermine their plans. Another plot could involve Regulus coming back into the wizarding world and having to deal with the changed perception of him, as everyone thought he was dead. He might also have to face his family again, especially Sirius, and deal with the complex emotions that come with it.
One interesting plotline could be about the characters' journey to discover the true meaning of love in a post - apocalyptic world. Maybe they find that love can exist even without all the trappings of modern society.
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.
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.
A plotline could involve Harry's discovery of his true parentage. Maybe he grows up thinking he's just James' son, and then finds out about Regulus. This could lead to a journey of self - discovery, as he tries to understand the parts of himself that come from both his fathers. He might also face opposition from some in the wizarding world who can't accept this unusual family situation.
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 plot could be her discovery of her father's past. Maybe she stumbles upon some old artifacts or memories that lead her to understand the sacrifices he made. For example, she might find his old Death Eater's mark and then try to piece together what it means for her family's history.
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.