One plot idea could be Itona's struggle to fully adapt to the classroom environment. He could start off as an outsider but gradually form deep bonds with his classmates. Maybe he discovers a new hobby or talent that helps him fit in better, like joining a music club in the school within the Assassination Classroom world.
Itona in the Assassination Classroom fanfic is often depicted as a complex character. He has his own unique backstory and growth arc. His relationship with other characters, like Nagisa and Karma, can be explored in different ways in fanfics. Sometimes he is shown as a more vulnerable side, which is different from his tough exterior in the original anime.
A plot could revolve around Itona getting into some trouble outside of school, perhaps with some people who are after his powers. Karma finds out and decides to help him out of his own sense of justice and also because he has come to care about Itona. They go on a sort of adventure where they have to hide from these pursuers and at the same time figure out how to permanently get rid of the threat. Along the way, they have deep conversations about their pasts, what they want in life, and this strengthens their bond.
Itona and Nagisa could be an interesting pairing. Nagisa's calm and understanding nature could balance out Itona's more impulsive side. They could have adventures together in the classroom, like working on a secret project to surprise the teacher.
A plot could revolve around a time - travel aspect. A student accidentally travels back in time to before Koro - sensei was experimented on and tries to change the course of events. This would involve a lot of moral dilemmas as they might be changing the future for better or worse.
Love interests can be a theme as well, although not as common as the others. Some fanfics might pair Itona with another character and explore how their relationship develops. This could be a romantic relationship or just a deep, platonic bond. It adds a different layer of emotion to the story and shows a different side of Itona's personality.
One idea could be that the main character uses his unique skills from the Assassination Classroom to protect the girls in his harem from other threats outside of the classroom. For example, a new group of assassins comes to town and he has to keep his harem safe while also dealing with school life.
Well, perhaps a group of students time travel to a future where they see the consequences of not assassinating Koro - sensei in time. This could give them new motivation and strategies when they return to their own time. Or, what if Koro - sensei time travels to different points in the students' lives and helps them in ways that are not related to assassination, like helping a student deal with a family issue in the past.
One idea could be the children of the original students having to face a new threat in the classroom. Maybe a relative of Koro - sensei who has a different teaching style but also a hidden, dangerous side.
One idea could be a new student arriving at Paradise High with a special ability that both helps and complicates the assassination attempts on Koro - sensei. For example, they might be able to sense his weak points more acutely but also have a personal vendetta against the students trying to kill him.
Another idea is a time - travel fanfic. Nagisa and Karma are sent back in time to when they first met. They remember all the things they've been through together in the future. This time, they decide to change some small things in their past interactions. They start to encourage each other more from the very beginning. Karma helps Nagisa to be more confident in his abilities, and Nagisa shows Karma the importance of thinking things through carefully. This changes the whole dynamic of their relationship in the present when they return to their original time.