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.
In such a fanfiction, Sirius could receive a howler that reveals some family secrets. The howler might be a way for his family to try and control him or bring him back into the fold. It could also lead to some internal conflict within Sirius as he has to deal with the public humiliation of the howler and what it says.
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.
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.
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.
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.
She could inherit some of Sirius's traits. For example, his rebellious streak might make Hermione more willing to break the rules when it's necessary to achieve a greater good.
Well, Sirius might be shocked by the Dursleys' treatment of Harry. He could try to rescue Harry from their mean ways.
In a fanfiction where Petunia meets Sirius, it could be a very interesting encounter. Sirius, being a wizard from the wizarding world, might shock Petunia at first. He could show her some simple magic, like making a flower bloom, which would both terrify and fascinate her.
In a Harry Potter Umbridge trial fanfiction, Umbridge could finally be held accountable for her cruel actions at Hogwarts. Maybe she'd be tried for her abuse of power, like using the blood - quill on students. The Wizengamot might hear testimonies from students like Harry, Hermione, and Neville about the unjust punishments and the way she ran the school tyrannically.
A 'Harry Potter Malfoys trial' fanfiction could have many twists. For example, the evidence against the Malfoys might be fabricated by some who want to take over their wealth and status. The Malfoys could then fight back, with Draco using his intelligence to prove their innocence. Hermione might be involved as a legal advisor of sorts, using her knowledge of wizarding law. And Harry could be conflicted, as he has his own history with the Malfoys but also believes in justice. The trial could also expose the dark underbelly of the wizarding legal system, with corruption and bias playing a role.