Dumbledore might play a key role. He has a lot of influence in the wizarding world. He could use his position to pressure the Ministry into giving Sirius a trial. And Remus Lupin could also be of assistance. He knows Sirius well and could testify on his behalf.
Some of the Order of the Phoenix members could step in to help. For example, Moody, with his experience in dealing with dark wizards and the Ministry, could offer strategic advice. And Tonks, with her ability to change her appearance, could perhaps infiltrate places to find more evidence. Even Hagrid, who has a big heart, might be able to give some useful information about Sirius from the past that could be used to support the call for a trial.
Hermione would definitely be a great help. She's smart and resourceful, and could do a lot of research to find evidence. Also, Ron would be there, providing support and loyalty as always.
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.
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.
Some independent fanfiction blogs also host such stories. However, they can be a bit harder to find. You can start by searching for Harry Potter fanfiction blogs through search engines. When you find some, check if they have a search function on the blog itself and look for the 'harry gets sirius a trial' fanfiction there.
Fanfiction about these three can take many forms. For example, there could be time - travel fics where Harry goes back in time and meets Regulus and Sirius when they were younger. This could lead to all sorts of interesting plot twists, like Harry trying to prevent Regulus from joining the Death Eaters in the first place. Or it could be a fic where Sirius survives and he and Harry explore the secrets of the Black family together, including things that Regulus left behind. There are also fics that focus on the emotional bonds between the characters, showing how the different personalities of Regulus, Sirius, and Harry interact and influence each other.
Hermione would be a great help. Her vast knowledge of magic and books could provide the necessary information for building the planet, like the right spells and magical theories.
Dumbledore might also play a role. He has vast knowledge and experience. He could offer some wise advice or even provide a special magical item or spell that is crucial for the unicorn's healing.
Dumbledore might offer some guidance. He has vast knowledge and experience. He could tell Harry about the significance of different elements in a staff and how to combine them to make a truly powerful and unique magical tool.
Dumbledore might help in a spiritual or advisory way. Even though he's passed away, his wisdom could be passed on through memories or some magical means. Also, Hagrid could be a great help. He could assist in building areas suitable for magical creatures with his love and understanding of them.
His best friends Hermione and Ron would surely be there to help. Hermione with her vast knowledge of magic could research about the egg, and Ron with his loyalty would stand by Harry's side in any danger related to the egg.