A possible plot is that Sirius Black gets a tip - maybe from an unexpected source like a house - elf - that Harry Potter is being lured into a trap. Sirius races against time to reach Harry. When he does, Harry is in a wizarding duel with some powerful dark wizards. Sirius jumps into the fray, using his own powerful magic skills. He manages to deflect the dark spells aimed at Harry and then they both apparate away to safety. This could lead to a deeper bond between them and also new revelations about Sirius' past that help Harry in his fight against Voldemort.
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 is that a dark wizard targets Remus Lupin and Sirius Black because of their close relationship with Harry Potter. The fanfiction could focus on how they protect themselves and Harry, and how this situation brings out new aspects of their magical abilities and their bond with each other. They might have to seek help from some unexpected allies within the wizarding world to survive.
Perhaps Sirius Black's daughter has a special connection to Harry through a long - forgotten prophecy. They start to notice strange coincidences and shared dreams. As they investigate, they find out that their destinies are intertwined in a way that could change the wizarding world. It could involve a journey to find a lost magical artifact that holds the key to fulfilling the prophecy.
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 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.
A unique idea could be a fanfic where Sirius Black time - travels. He could go back to his school days and try to change certain events that led to his future troubles. This would be an interesting way to explore his character development in a different timeline.
Often, a common plot point is the struggle against the wizarding world's norms. Since their relationship is rather unconventional in the context of the original story, they face a lot of opposition. For example, in some fanfics, the Ministry of Magic tries to interfere.
Maybe Sirius in his puppy Animagus state gets caught by Filch. Filch tries to keep him as a new pet for Hogwarts, not realizing it's Sirius. Sirius then has to figure out a way to escape while also not revealing his true identity. This could lead to a series of hilarious events as Sirius tries to outwit Filch and his cat, Mrs. Norris.
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.