A possible plot could be that Sirius Black is in the hospital due to being poisoned. As he recovers, he overhears some nurses talking about a strange occurrence in the hospital's basement. Curious, he investigates and finds out that there are some dark forces at work, trying to steal powerful potions stored there. He then has to find a way to stop them while still recovering from his poisoning.
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.
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.
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.
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.
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.
A possible plot could be a time - travel story. Ginny accidentally travels back in time and meets a young Sirius Black. They team up to change an event in the past that has far - reaching consequences for the wizarding world in Ginny's time. Or perhaps Sirius, after his death, is able to communicate with Ginny in a way similar to how he did with Harry. He gives her advice on dealing with her relationship problems, like her feelings for Harry and how to handle jealousy among friends.
A great plot idea is that Sirius and Lily form an alliance to protect James Potter. Sirius, being James' best friend, and Lily, being James' future wife. They could have adventures together, facing off against Death Eaters even before the full - blown war starts. It could also be a story where Lily's kindness starts to rub off on Sirius, and he begins to change some of his more reckless ways. For example, he might start to study more seriously under Lily's influence. This could lead to some really interesting character development for both of them.
One plot idea could be that fem Harry Potter discovers a hidden prophecy that involves Sirius Black and they have to work together to figure out what it means. Along the way, they face challenges from dark wizards who also want to get their hands on the prophecy.
In a 'Harry Potter meets Sirius Black fanfiction', it could be interesting if Sirius and Harry found a way to time - travel. They could go back to the time when James and Lily were alive. This would not only allow Harry to meet his parents but also give Sirius a chance to change some events that led to his downfall. They might also interfere with the actions of Voldemort in the past, creating a whole new timeline and a different future for the wizarding world.
One idea could be Sirius teaching Harry all the secret passages around Hogwarts that he knew as a student. Another could be Sirius introducing Harry to the wizarding world's underground music scene. Or perhaps Sirius helping Harry deal with his nightmares about Voldemort by sharing his own experiences of facing dark magic.