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.
One interesting fanfiction idea could be that Sirius Black, after his death, is somehow brought back to life by a powerful magic. He then decides to use this second chance to directly oppose Voldemort. He teams up with other characters, like Hermione and Ron, to come up with a new strategy to defeat the Dark Lord. This could involve exploring new magical theories and spells that were previously unknown.
One possible fanfiction could be about Sirius Black being in a situation where he has to face Voldemort alone. Maybe he discovers a secret weakness of Voldemort that no one else knows. Sirius, with his brave and reckless nature, decides to take advantage of this knowledge to try and take down the Dark Lord once and for all.
One idea could be that Sirius Black takes his godson on a secret adventure through the Forbidden Forest. They encounter magical creatures and Sirius uses the opportunity to teach his godson about the importance of respecting all forms of magic.
It's possible in fanfiction, but it would be a huge departure from his character. Sirius Black is a Gryffindor, known for their courage and loyalty to the side of good. He has personal reasons to hate Voldemort, like the death of his friends at the hands of the Death Eaters. But fanfiction authors might create a complex backstory to justify his joining. Maybe they could write that he was under a powerful spell or was tricked into believing Voldemort had different intentions. Still, it would shock most fans who know the original Sirius.
One could also write a fanfiction about Sirius Black's impact on Hogwarts during the events of Prisoner of Azkaban. For instance, his presence, even though he's on the run, could have a big influence on the students. Some students might start to question the Ministry of Magic's actions, and Sirius could inspire them to fight for justice.
One idea could be that Sirius Black's daughter, who has been hidden away all this time, discovers her true identity while trying to break into Azkaban to clear her father's name. She could have some unique magical abilities that help her in this dangerous quest.
One idea could be that Sirius uses his knowledge of magic from his time as an Animagus and as a member of the Order to quickly heal Hermione's injury. Maybe he transforms into his dog form to track down any threats still lurking around her.
A popular plot device is the discovery of a 'hidden truth'. For example, Sirius might find out that there is some sort of family connection or prophecy that makes him think he should be on Voldemort's side. Another common one is the use of a love interest. Maybe a Death Eater or someone close to Voldemort seduces Sirius into joining. And sometimes, it could be a situation where Sirius is desperate and feels that Voldemort is the only way to achieve a certain goal, like saving someone he loves who is in danger from another source.
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.
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.
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.