A new student could be the main character. This student comes to Hogwarts and stumbles upon the diary. They don't know about the diary's dark past. As they start to write in it, they get drawn into a world of mystery. They have to rely on their new friends at Hogwarts to help them figure out what's going on with the diary and how to deal with the strange things that start to happen.
Neville could also be a main character. He's often overlooked but has a lot of potential. In a diary fanfiction, Neville could find the diary and it could give him the confidence he needs. He might discover some hidden magic in himself through the diary's influence, and it could be a story of his growth and transformation.
Hermione could be a great main character. She's curious and intelligent, so she would be really interested in exploring the secrets of the diary. She could use her vast knowledge of magic to try and figure out how the diary works and what secrets it holds.
A lesser - known character like Ernie Macmillan from Hufflepuff could be the main character. As a prefect, he could bring out the unique qualities of Hufflepuff house. He might have to deal with issues like other houses underestimating Hufflepuff students. His story could focus on how he promotes the values of hard work, loyalty, and fairness within the house and the school as a whole.
Harry Potter himself could be the main character as a revenant. Maybe he has some unresolved issues from his past, like his relationship with his parents. His revenant form could be on a journey to fully understand their sacrifice and his own place in the wizarding world's history.
It could be a younger sibling of one of the main characters in the original series. For example, Ginny's little sister who has big shoes to fill but also a great passion for Quidditch.
Hermione could be a great main character in an elementalist fanfiction. Her intelligence and thirst for knowledge would make her eager to learn about and master elemental magic. She could be the one to discover a long - lost elemental spell book and start exploring the powers.
It could be a new student at Hogwarts who has a natural talent for dealing with beasts. This student could be from a family that has always had an affinity for magical creatures but has kept it a secret until now.
A Muggle - born mouse. It could be a unique concept where a mouse from the Muggle world somehow ends up in the wizarding world. It would be interesting to see how it copes with all the magic and the rules of the wizarding society.
Neville could also be a main character in this type of fanfiction. He could stumble upon a clockwork - related mystery in the Hogwarts gardens. Maybe there are some clockwork - powered plants that are acting strange. Neville, with his new - found confidence, takes it upon himself to solve the mystery and in the process, uncovers a deeper secret about Hogwarts and its connection to clockwork magic.
A young witch who was raised by dragons after being lost in the mountains as a child. She returns to the wizarding world as a dragonspeaker and has to adapt to human society while also using her skills to protect both the dragons and the wizards. She could be the main character as she bridges the gap between the two worlds.
How about Neville Longbottom? He could be the main character in a fanfiction set in the Chamber. Neville, who has always been underestimated, could show his true courage and strength. He might discover a hidden ability that helps him face the challenges in the Chamber, perhaps something related to his connection with plants, which could play a crucial role in defeating the Basilisk or whatever new threat is in the Chamber.
Neville is a likely candidate. He has always been brave and has a strong connection to the Gryffindor values. If not for Harry, Neville could have easily stepped into the role of the main character. His growth throughout the series shows that he has the potential to be a great hero.