The fanfiction could start with Sirius being magically transported to a new, more just courtroom. Dumbledore could be secretly working behind the scenes to ensure a fair trial. The trial could be full of twists and turns. For example, the dementors that were involved in his original imprisonment might be called as witnesses, but Sirius could use his knowledge of their weaknesses to discredit them. And in the end, he is proven innocent, much to the joy of his supporters and the shock of his enemies.
In such a fanfiction, perhaps Harry discovers some new evidence that was overlooked. He might find a witness who was too afraid to come forward before. Then he presents this evidence to the Ministry, forcing them to give Sirius a trial.
Well, Sirius might face a series of challenges during the trial. He could be wrongly accused and have to fight hard to prove his innocence. Maybe his friends would come to his aid, bringing new evidence or testifying on his behalf.
Obviously, Sirius himself is the central figure. His friends like Harry, Hermione, and Ron could be involved. They might be in the courtroom, trying to support Sirius in any way they can. The Ministry of Magic officials would also be there, as they are the ones conducting the trial.
One common theme is injustice. Since Sirius was wrongly accused, fanfictions often explore how unfair the trial was. Another theme is loyalty, as his friends' loyalty to him is shown in different ways. For example, they might try to find evidence to clear his name.
One really good one I read had a detailed courtroom scene where Harry used some really clever magic to present evidence. It was on FanFiction.net. Another great one on Archive of Our Own had a unique take on how the wizarding world politics played into Sirius' trial.
The plot could involve Sirius Black's child being raised in the Muggle world by a relative who doesn't want anything to do with the wizarding world. However, as the child turns 11, strange things start to happen. A wizard or witch arrives to take the child to Hogwarts. Once at Hogwarts, the child starts to learn about their father and the rumors surrounding him. They then decide to clear their father's name and along the way, make new friends who help them in their quest.
Well, the plot could involve Sirius trying to protect his kid from the dark forces that are still after him. Maybe his kid has some special powers, like being able to communicate with magical creatures easily. And Sirius has to teach the kid how to control and use these powers without attracting too much unwanted attention.
Obviously, Sirius Black himself is a key character. He's the one on trial, so he'll be at the center of the story. Then there's Harry Potter. He might be very involved, trying to prove Sirius's innocence. Hermione Granger could also be important, using her research skills to dig up evidence. And Remus Lupin, as Sirius's old friend, would be a crucial witness.
The plot could involve Sirius Black coming back as a vengeful spirit. After his death, his soul is not at peace. He haunts the halls of Hogwarts, especially the areas that held bad memories for him. Students start to see his apparition, and strange things begin to happen. For example, portraits on the walls start to cry or rooms suddenly become freezing cold. The main character, a young student, decides to find out the truth behind these hauntings and has to face the terrifying Sirius Black's spirit.
One possible plot could be that James Sirius Potter has done something really naughty at Hogwarts, like pulling a major prank that went too far. The Howler then arrives, screaming at him in front of all his friends and classmates, causing him great embarrassment. This could lead to him having to face the consequences and also learning a valuable lesson about the limits of his mischief.