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.
Hermione could be a main character. She is always by Harry's side and her intelligence would be crucial in any discovery. She might be the one to figure out the clues or research the background of the discovery.
Ron could also be the main character. He has his own courage and loyalty. He could be the one who has an unexpected connection with the poisonous snake, like being able to communicate with it in a special way that no one else can.
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.
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.
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.