Hermione could be the main character. Since she's so studious and knowledgeable, she would be the first to notice something amiss during the Halloween events. She would then use her vast knowledge of spells and magic history to try and stop the disaster from getting worse, all while trying to keep Ron and Harry in line.
It could also be a new student. This new student arrives at Hogwarts just in time for Halloween. They accidentally trigger the disaster and then feel responsible for fixing it. Throughout the story, they learn about Hogwarts, make new friends like Harry, Hermione, and Ron, and grow as a wizard while trying to undo the mess they've made.
Perhaps in this fanfiction, there's a dark object that gets activated on Halloween. It could be a cursed item that was hidden away but some curious students find it. As a result, it starts to spread a strange magic throughout the school, making everyone experience their worst nightmares related to the Forbidden Forest, like seeing thestrals and acromantulas everywhere, and the teachers have to figure out how to stop it.
Obviously, Harry Potter himself could be a main character. He has the courage and magical ability to handle the strange and spooky situations in Halloween Town. Hermione Granger would also be a great main character as she has the knowledge to figure out the unique magic and lore of the town. Ron Weasley could be there too, providing some comic relief and loyalty. Another possibility could be Luna Lovegood. Her out - of - the - box thinking would be very useful in a place as strange as Halloween Town.
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.