A plot could revolve around Narcissa and Sirius in their Hogwarts days. They could be involved in a mystery within the school, like a series of strange disappearances of students. Since they are both from the Black family, they might have some inside knowledge that helps them solve the mystery, and in the process, form an unexpected bond.
Bellatrix Lestrange could be a great addition. Her extreme loyalty to Voldemort and her relationship with Narcissa as sisters, and her hatred for Sirius as a blood traitor, would add a lot of tension to the story.
Another plot could be a time - travel fanfiction. James Potter somehow travels to a time when Narcissa is young and in a difficult situation. He decides to help her, and in the process, they form an unexpected bond. This bond then has consequences for the future as James returns to his own time.
Well, perhaps there's a fanfiction where Sirius Black is wrongly accused again, but this time Hermione believes in his innocence from the start. She uses her intelligence and resources to prove his innocence while they are on the run from the Ministry. Along the way, they form a deep bond and discover new magical abilities within themselves that they use to clear Sirius' name and also make some changes to the wizarding world's justice system.
Well, he could be in the hospital because of a curse. While there, he starts to form a bond with a young patient who has no family. Sirius then decides to protect this child and in the process, uncovers a plot against the wizarding world that has some connection to the hospital. Or perhaps Sirius is in a coma and his friends use magic to enter his mind in the hospital. They find out things about him they never knew before.
One idea could be a humorous take on Sirius as a baby with his magical abilities already showing in unexpected ways while in diapers. For example, he might accidentally levitate his diaper when he gets excited.
Maybe a story where Sirius Black fakes an illness to get close to Madam Pomfrey because he suspects she has some information about his family. But as he spends more time with her in the hospital wing, he starts to feel guilty for deceiving her. Meanwhile, Madam Pomfrey starts to suspect that Sirius is not as ill as he seems, and their relationship becomes a game of cat - and - mouse, filled with tension and unexpected moments of kindness.
Well, a plot could involve Harry being raised by Narcissa Black in a world where the wizarding world is more divided than ever. Harry, influenced by Narcissa, has a different view on blood purity. He could be trained in dark arts defenses in a more Slytherin - like way. This would make his battles against Voldemort very different. He might also have a closer relationship with Draco from the start, and they could end up as allies rather than rivals. Plus, Narcissa's influence could lead Harry to have a more nuanced understanding of the Death Eaters, which would add a lot of complexity to the story.
A fanfiction could center around Sirius Black's attempts to rescue someone from Voldemort's clutches. He uses his skills as an Animagus and his knowledge of the wizarding world's hidden paths to get close to Voldemort's lair.
In a fanfiction, Sirius could have his hair cut by someone else as a sign of trust. For example, he might let a new ally cut his hair, and during the process, they could have a deep conversation about his past, his hopes for the future, and his relationship with Harry. This haircut then becomes a bonding moment between them.
One plot idea could be that Sirius, who has a connection to Ginny through his friendship with the Weasley family, starts to mentor Ginny in some magical skills. He could share his knowledge of the more dangerous aspects of magic, like dark wizard - fighting techniques, which Ginny is eager to learn. This could lead to a strong bond between them as they train together.