Hermione could also be the main character. Given her thirst for knowledge, she might stumble upon the Throne of Heroes concept while researching in the library. She could then take it upon herself to understand it fully and use it to better the situation in the wizarding world, whether it's by bringing in new forms of magic or new allies to help in the fight against evil.
Harry Potter himself could be the main character. He could be the one who discovers the connection between his world and the Throne of Heroes. He might then use this knowledge to gain new allies and powers to fight against the dark forces in his world.
One possible interesting plot could be a crossover where the heroes from the Throne of Heroes are summoned into the Harry Potter world. They could use their unique powers and abilities to help in the fight against Voldemort. For example, a heroic archer from the Throne of Heroes could provide long - range support during battles at Hogwarts.
Obviously, Harry Potter himself would be a main character. He has the most experience in fighting dark wizards.
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.
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.