The teacher could be a main character as well. They could be a strict but fair instructor who challenges the students to reach their full potential in writing. Or perhaps a young and inexperienced teacher who is still learning how to handle the classroom environment while also inspiring the students to write great compositions. Also, there could be a student who is a natural leader among the classmates when it comes to writing projects, and this student could be a main character.
There are other possibilities too. For instance, a visiting author who comes to the composition classroom could be a main character. They could bring new ideas and inspiration to the students. Or it could be a janitor who has a hidden love for writing and secretly encourages the students. Even a pet that belongs to someone in the classroom could be a main character in a more whimsical fanfic, influencing the writing atmosphere in some way.
Karma Akabane would also be a likely main character. His mischievous and intelligent nature would have evolved over the years. He could be involved in some sort of high - stakes international work, using his sharp mind and combat skills. And of course, he might still have that competitive edge when it comes to his former classmates.
Well, a lot of things really. Maybe a plot where the students compete in a writing contest within the classroom setting. Or perhaps a scenario where the classroom itself has some sort of magical or special property that affects the writing process. Another possibility is creating a story about the relationships between the students and how they develop through their writing experiences in the composition classroom.
The children of Nagisa and Kayano could be main characters. They might inherit some of their parents' skills and quirks. For example, Nagisa's son could have his father's stealthy nature.
Koro - sensei would likely be one of the main characters. His unique abilities and his role as the target of assassination in the original 'assasination classroom' would also be important in the infinite loops scenario.
Ayanokoji could be one of the main characters. He's very intelligent and mysterious, and he might fall in love with a female character like Horikita, who is also smart and strong - willed.
A new transfer student from a foreign assassin school. This character could bring in different assassination techniques and a different cultural perspective on dealing with Koro - sensei. They might be more ruthless or have a unique code of ethics.
In the 'spy classroom novel', the main characters are likely to be the trainees in the spy classroom. Some may be the best in combat, some might be excellent at decrypting codes. There could also be a teacher or instructor figure who guides them through their training and missions. These characters interact with each other, and their relationships play an important role in the story.
I'm not sure exactly, but probably there are students who are the main heroes.
Nagisa Shiota is also very popular. Fanfics could focus on his growth as an assassin and how his initially meek personality evolves over time. His unique skills and his friendship with other classmates can be great elements to build a story around.
Ayanokoji Kiyotaka is often a popular choice. His mysterious nature and high intelligence make him a great character to explore further. Writers can delve into his past or create new situations that test his abilities.