Luna Lovegood might also be a possible love interest. Her unique personality and view of the wizarding world could attract Harry in a different way. She's not afraid to be herself and is very open - minded. In an AU, Harry could see her in a new light and they could start a relationship based on their shared experiences of being a bit different from the norm in Hogwarts.
In AU 4th - year fanfictions, relationships can change a great deal. For example, if Harry was raised by Sirius instead of the Dursleys. His confidence might be higher, and his relationship with Draco could be less antagonistic from the start. Maybe they could even become reluctant allies in the face of a new threat at Hogwarts. Also, Hermione might form a closer bond with Neville as they study together more often in this AU, and Ron could become a bit jealous of their newfound friendship.
One underrated character who could have a bigger role is Seamus Finnigan. He's always been on the sidelines a bit, but in a 7th year au, he could be a key figure in the Gryffindor resistance against the Death Eaters at Hogwarts. His skills in charms and his Gryffindor courage could be put to great use. Also, Professor Trelawney could be more important. She might have a real, accurate prophecy in this au that she shares with Harry. Her knowledge of divination could be the key to understanding some crucial aspect of Voldemort's downfall, rather than just being seen as a bit of a joke like in the original story.
A great antagonist could be a corrupted teacher. For instance, a teacher who is secretly working for Voldemort but has been hiding in plain sight at Hogwarts for years. They could manipulate the Triwizard Tournament to their own ends, causing all sorts of trouble for Harry and his friends.
One interesting plot twist could be that in the AU, Neville Longbottom is the one secretly chosen for the Triwizard Tournament instead of Harry. It would completely change the dynamic as Neville has his own insecurities and strengths that would play out differently in the challenges.
One idea could be that Harry discovers a new, hidden magical ability within himself that helps him in the final battle against Voldemort. For example, the ability to communicate with the spirits of Hogwarts founders for guidance. Another could be that the relationships between the characters take a different turn. Maybe Hermione and Ron have a huge fight early on and Hermione teams up with Draco for a while out of necessity, which changes the dynamics of the whole group. Also, Neville could have an even more prominent role, leading a secret group of students who are training in a new form of magic that they hope will give them an edge in the fight.
Well, a cool plot twist might be that in this 4th - year AU fanfic, the Triwizard Tournament tasks are all designed by a rogue Death Eater who has infiltrated Hogwarts' staff. So the tasks are not just tests of courage and skill but also traps to try and harm the students. Also, Harry might discover that his parents were actually part of a secret society within the wizarding world that was fighting against a different dark force, not just Voldemort, which changes his whole perspective on his family and his role in the wizarding world.
A half - vampire student. This character could have unique abilities like super speed and enhanced senses. They could be a bit of an outcast, but also have a mysterious allure and help Harry in unexpected ways, perhaps in dealing with the Dementors.
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.
Obviously, Harry could still be a main character. He'd be the brave new kid in high school, trying to make friends and figure out his place. Hermione would also be there, the super - smart girl who is always acing her classes and getting involved in every club. Ron could be Harry's best friend, the laid - back but loyal guy. And Draco could be the rich, snobby kid from the 'popular' group who constantly gives Harry a hard time.
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.