The family members of the main characters can be a great focus too. Their reactions to the events in the fanfic, their influence on the main character's decisions, and how their relationships change over the course of the story can be really engaging to write about.
I don't know specifically as I haven't read the book, but usually in a novel like this there could be a protagonist, perhaps a young person, and other supporting characters like friends, family members or love interests.
I don't know as I haven't read it. There could be a young protagonist, perhaps a teenager, and maybe some friends or family members that play important roles.
One idea could be exploring the backstories of the minor characters in more detail. For example, the local fisherman who always gives the main character a friendly nod. Another could be a time - skip scenario where the characters reunite after a long time apart.
Belly is always an interesting character to focus on. Her journey of self - discovery and her evolving relationships with Conrad and Jeremiah offer a lot of material. You can explore her insecurities, her growth, and how she navigates love and friendship.
Heathcliff is an obvious choice. His complex personality, filled with love, hatred, and a thirst for revenge, offers a lot of material for a fanfic writer. You can explore his past more deeply or give him a different future.
Serafina Pekkala could be an interesting character to focus on in a 'a subtle knife fanfic'. As a witch, she has her own set of powers and a unique perspective on the events. You could write about how she views the Subtle Knife and its implications for the different worlds. Also, Iorek Byrnison, the armored bear. His strength and loyalty could be a great addition to a fanfic. He could be involved in protecting the knife or helping the characters who possess it in their quests.
Todd Hewitt is an obvious choice. He's the main character in the original story, so there's a lot of room to expand on his character, his relationships, and his journey. Viola Eade is also great. You could explore her experiences more deeply, especially her interactions with Todd and how she adapts to the new world she finds herself in. Another good character is Mayor Prentiss. He has a complex personality, and writing about his backstory or what could have happened to him after the events of the original story can be really interesting.
The Suriel is a character that has a lot of potential in a fanfic. It has a strange and somewhat otherworldly presence. You could write about its role in the world from a different perspective. Maybe it has more knowledge than it initially lets on and could be a key player in a different kind of story. And Lucien, with his torn loyalties and own journey of self - discovery, is also a good character to center a fanfic around.
Steven is an underrated character. He has his own story with his friends and his relationship with Shayla. Fanfiction could focus on his growth during the summers, like how he deals with his sister's love drama while also having his own. Another could be Taylor. She's Belly's best friend, and a fanfic could explore her perspective on Belly's love life and her own experiences in the beach town.
Yuki is a very popular character to focus on. She has a complex backstory, being raised among vampires and having a special connection to both the vampire and human worlds. Her relationship with Zero and Kaname also offers a lot of material for fanfic writers to explore, such as her inner conflict between her love for both of them and her duty to maintain the peace at Cross Academy.