A really interesting plot might be that Regulus Black time - travels back to the days when Gryffindor was still alive. He gets to interact with Gryffindor and learns about the true essence of the Gryffindor house. This experience changes his perspective on life and makes him want to change the future of his family in Slytherin. He then tries to bring some of Gryffindor's values back to his own time, which causes a stir among the pure - blood families in Slytherin.
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.
Another plot could involve Regulus Black being sorted into Gryffindor instead of Slytherin due to a magical mishap. He then has to deal with the prejudice from some Slytherins and also the pressure to live up to Gryffindor ideals. He could form an unlikely friendship with a group of Gryffindors and together they discover a hidden secret about Hogwarts that was somehow related to his family's past in Slytherin.
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 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 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.
It could be a power - hungry Slytherin who doesn't like that Regulus is showing an interest in Gryffindor values. This Slytherin might see it as a betrayal to their house and try to stop Regulus by any means necessary, even if it means using dark magic.
Another plot could be that Draco's sorting into Gryffindor causes a rift in his family. His parents are disappointed, but Draco finds a new family among his Gryffindor housemates. He could also have a love interest in a Gryffindor girl, and they have to overcome the house rivalries to be together. The story could explore how Draco changes and grows in this new environment.
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 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.
One plot idea could be that she struggles with the family expectations of being a Malfoy while having the brave and chivalrous values of Gryffindor. Maybe she has to keep her Gryffindor identity a secret from her family at first. Another could be that she forms an unlikely friendship with a pure - blood supremacist in Slytherin and tries to change their views through her Gryffindor - influenced actions. And she might also be involved in a quest to protect Hogwarts from a dark force, using her unique position of being a Malfoy yet having Gryffindor qualities.
A plot could be that a Gryffindor Animagus accidentally reveals their form during a Quidditch match. This causes chaos as everyone tries to figure out who it is. It also draws the attention of the Ministry of Magic who have strict rules about unregistered Animagi. The character then has to deal with the consequences while also trying to prove they mean no harm.