Well, in the 5th year trial fanfiction, Harry might face some unjust accusations. Maybe the Ministry of Magic is trying to pin something on him again, just like in the original story. He could be put on trial for using magic outside of school when he was actually defending himself.
Obviously, Harry Potter himself is a key player. He is the one on trial and has to defend himself. Hermione and Ron would also be important. They would be by Harry's side, helping him gather evidence and coming up with arguments to prove his innocence.
Another interesting plot could be that Harry's connection to Voldemort causes some strange magical disruptions during the Triwizard Tournament. These disruptions seem like accidents at first, but Harry gradually realizes they are being caused by his own dark - side related powers that are being awakened in his fourth year.
Make sure the character's actions are consistent with their personality. If they are shy, they won't suddenly be the center of attention in the Great Hall without a good reason. Their growth throughout the fanfiction should also be gradual. In the fourth year at Hogwarts, there are a lot of new challenges like the Triwizard Tournament. So, the character should respond to these challenges in a way that is in line with who they are.
It could be about a trial within the Harry Potter universe created by a fan. Maybe it's a trial of a character for a crime they're wrongly accused of, like Harry being put on trial for using magic outside of Hogwarts again. It might explore new aspects of the wizarding legal system and how characters deal with it.
The mystery surrounding Voldemort's return is also a common theme. Fanfictions often explore how the students and teachers at Hogwarts start to suspect and then deal with the news. Additionally, the theme of identity is prevalent. Harry is constantly trying to figure out his place in the wizarding world, especially with all the new attention from the tournament and the rumors about him and Voldemort.
One great fanfiction idea could be Harry dealing with the Triwizard Tournament challenges in unique ways. Maybe he discovers a hidden power within himself that helps him out during the tasks, like a new form of magic he didn't know he had.
Well, if the trial is about a Hogwarts student who is in trouble, then Neville could be one of the main characters. Or if it's a trial related to the dark side, Draco Malfoy could play a significant role. It's also possible that some minor characters from the original books, like Luna Lovegood, could become main characters in this fanfiction if they are somehow closely related to the trial, for example, if they have crucial evidence or are the ones being accused.
I'm not sure specifically as there could be many different plots in fanfictions about this. But generally, it might involve a trial where Veritaserum (the truth - telling potion) is used in the Harry Potter universe. Maybe a character is wrongly accused and the Veritaserum is crucial in revealing the truth, or perhaps it's misused in the trial, causing chaos and unexpected revelations.
Harry, Ron, and Hermione are almost always main characters. They continue their journey together in the 8th year. Ginny Weasley also often plays a significant role, especially in relation to Harry. Neville Longbottom might be another important character as he has proven himself during the war and could be involved in rebuilding Hogwarts.