In addition to the main trio from the Harry Potter series, there could be some of the Hogwarts professors as main characters. For example, Professor Dumbledore might have some knowledge about the balverines that he imparts to Harry. And Hagrid, with his love for all kinds of strange creatures, could be involved in trying to tame or at least understand the balverines better.
The plot might involve a crossover where the magical world of Harry Potter collides with the concept of balverines. Maybe Harry and his friends encounter balverines during one of their adventures outside of Hogwarts, and they have to use both their magic skills and newfound knowledge about balverines to survive.
Neville Longbottom could be a main character. He has always had the potential for great things and in a world without Harry, he would be likely to step into the spotlight. He has the courage and the connection to Hogwarts that could make him a central figure in the fight against evil.
Hermione could also play a role. Given her intelligence and love for learning, she might take an interest in accounting to better manage the finances of the various causes she's involved in, like the Society for the Promotion of Elfish Welfare. So she could be a main character or at least a supporting one in this fanfiction.
Obviously, Harry Potter himself could be a main character. He could be the one who is initially reluctant to participate in a dance event but then discovers a new passion for it. Hermione could also be a main character as she might be the one researching the history and significance of different wizarding dances.
Harry, Hermione and Ron could be the main characters, exploring the mysteries of the mansion. Or perhaps a new generation of wizards, like the children of the original trio. Neville Longbottom could also be a main character, using his knowledge of plants to uncover secrets in the mansion's garden.
Even Neville Longbottom's descendants could be interesting main characters. Maybe his grandson has a unique talent for herbology, like Neville did. He could use his skills to uncover a plot that involves poisoned plants in the Hogwarts gardens and throughout the wizarding world, and along the way, he becomes a hero in his own right.
The main characters would be Harry Potter and King Arthur. But I think the character of Guinevere could also be important. She might be intrigued by Harry and his different kind of magic. Additionally, Sirius Black could be written into the story. He could be a sort of spirit guide for Harry in Camelot, giving him advice from the afterlife on how to deal with the new and strange situations he finds himself in.
Draco Malfoy could be a main character in a wedding fanfiction. He could be marrying a witch who helps him redeem himself after the events at Hogwarts. Their wedding could be a sign of his new start in life. It might be held in a more secluded area of the Malfoy Manor, renovated to be less dark and foreboding, with a mix of old pure - blood traditions and new, more inclusive wizarding practices.
A Hogwarts professor who has knowledge of both the wizarding world and some dark secrets related to other dimensions. This professor could play a key role in dealing with the Arrancar, whether it's trying to communicate with it or stop it from causing harm.
I think Luna Lovegood could also be a main character. She has such an unique way of thinking that she might bring a different perspective to the math concepts in the fanfiction. Maybe she could find a connection between her strange beliefs and math that no one else has thought of before. And she could use this knowledge to help her friends in some magical math adventures.