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.
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.
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.
One interesting plotline could be Harry dealing with the aftermath of the war in Hogwarts. He might be trying to heal the school and its students from the trauma. Another could be his relationship with Ginny evolving further as they face new challenges together in their final year.
One interesting plotline could be exploring more of Snape's inner turmoil during Harry's 6th year. Maybe showing his secret efforts to protect Harry while still maintaining his cover among the Death Eaters.
Hermione is also a favorite. In fanfiction, she can be shown delving even deeper into her studies, perhaps uncovering secrets about the wizarding world's history that are relevant to the 6th year events. Also, her relationship with Ron and how she deals with her own feelings and insecurities is a popular theme. Ginny is another popular character. Fanfictions might focus on her as a powerful witch in her own right during Harry's 6th year, showing her skills on the Quidditch pitch and her influence among the students.
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.