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.
Ginny Weasley is sometimes overlooked in terms of her potential in the 7th year. In an au, she could be leading her own group of students in secret dueling lessons to prepare for the final battle. Her experience with the diary Horcrux and her own strong magic could make her a real force. Additionally, Dean Thomas could have a more prominent role. He has a creative side and in the au, he could be using his art skills to create magical maps or symbols that help the resistance move around Hogwarts undetected. He could also be a great source of moral support for his friends during the difficult 7th year.
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.
Winky the house - elf. She was briefly in the fourth year story but there's so much more that could be done with her. Maybe she has some secrets about the Dark Arts that she's been hiding because she's afraid of getting in trouble.
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.
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.
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.
Seamus Finnigan could be a great focus. He's often seen as just a side character, but in fanfiction, he could have his own moment. Maybe he discovers a new spell related to the Triwizard Tournament tasks that no one else knows about. He could use this spell to either help Harry or get himself into trouble while trying to prove himself.
Definitely Firenze. He made an appearance in this book, but in fanfiction, he could be more involved. Since he has unique insights as a centaur, he could play a key role in helping Harry understand the prophecies or the true nature of the events related to Sirius. Also, consider Madam Pomfrey. She's often overlooked, but she could be a great source of knowledge about past students and events at Hogwarts that might be relevant to the situation with Sirius.
Neville Longbottom is often underrated. In a fanfiction, he could have a more central role within the castle. Maybe he discovers a hidden herb garden in the castle grounds that has magical plants with the power to heal any wound. Another could be Luna Lovegood. She could find a secret room in the castle filled with strange and wonderful magical creatures that only she can communicate with.
One underrated character for Harry Potter OotP fanfiction is Seamus Finnigan. He had his own doubts about Harry in the original story. You could expand on that. What if he had a change of heart in a more in - depth way? Maybe he discovers something that makes him fully trust Harry again. Also, Parvati Patil could be a focus. She could have her own side story about her Divination skills and how they might play a role in the bigger battle against the Dark Lord.