Remus Lupin is often present. He was one of Sirius' old friends and a fellow Marauder. Their friendship can be delved into deeper, especially considering the complex history between them. Dumbledore is another common character. He plays an important role in the wizarding world and his interactions with Sirius can add depth to the story.
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.
In many fanfictions about Sirius Black in Prisoner of Azkaban, he is often given a chance for a new adventure. Writers might imagine him escaping Azkaban in more elaborate ways, or having different interactions with Harry Potter and his friends. For example, some fanfictions have him forming a secret alliance with other prisoners to break out together.
One great fanfiction could be 'Sirius's Redemption'. In this fic, Sirius, after escaping from Azkaban, tries to reintegrate into the wizarding world while dealing with his past traumas. It delves deep into his relationships with Harry and his old friends. The author does an amazing job of portraying Sirius's struggle with his sanity after years in that dark prison. Another interesting one is 'Sirius: A New Beginning'. It focuses on Sirius starting a new life outside of the shadow of Azkaban, exploring new magic and forming new alliances. It's a really engaging read if you like seeing the character grow in different directions.
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.
He could be constantly thinking about his godson, Harry. The thought of seeing Harry again and protecting him from Voldemort would be a huge motivation. Sirius would probably replay in his mind all the things he wants to teach Harry and all the ways he can be there for him. This would keep his mind focused on the future outside Azkaban and give him the determination to escape.
A good Sirius Black post - Azkaban fanfiction should first of all capture his character accurately. He has been through a great deal in Azkaban, so his mental state should be reflected realistically. For example, his paranoia and his moments of confusion. Also, the relationship with other characters like Harry, Hermione and Ron is crucial. If it's a story where he tries to be a part of Harry's life again, it should show the complexity of that relationship. Good use of the wizarding world's lore is also important. Maybe he discovers some old magic related to his family while trying to start anew. And, of course, an engaging plot that keeps the readers hooked from start to finish.
She could be seeking revenge against those who wronged her father. So she goes to Azkaban, using her father's old contacts and knowledge, to find the real culprits. She might be a powerful witch, with skills in Occlumency to protect her mind from the dark influences in Azkaban.
In some 'sirius black after azkaban fanfiction', there could be a plotline where Sirius has to deal with the stigma of being an escaped convict, even though he was wrongly imprisoned. He might face distrust from some in the wizarding community. At the same time, he could be on a journey to find out who really betrayed his friends all those years ago, which leads him on an adventure full of mystery and danger.
In some fanfictions, Harry might use powerful magic he's learned secretly. For example, he could discover a new spell that can break the Azkaban's defenses. He then sneaks into Azkaban, locates Sirius, and uses this spell to free him from his cell. Maybe he also has to fight off some Dementors on the way out.