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 plot idea is that a rival class at Paradise High emerges, also aiming to assassinate Koro - sensei. Class 3 - E then has to deal with the competition, and there could be some interesting rivalries and alliances formed between the students of the two classes.
Maybe there could be a school festival at Paradise High in the fanfic. The students of Class 3 - E have to balance their preparations for the festival with their assassination plots. They could use the chaos of the festival to set up elaborate traps for Koro - sensei, like disguising a stage prop as a weapon.
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.
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.
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.
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.
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.
One plot idea could be Okuda developing a new type of anti - Koro - sensei formula that has unexpected side effects. Maybe it makes her able to understand Koro - sensei's language in a deeper way, leading to some comical yet heartwarming interactions between them as she tries to communicate with him privately about the students' progress. Another could be a story where Okuda has to team up with a rival classmate who initially doubts her skills, but as they work together on a project related to Koro - sensei, they learn to respect each other and Okuda shows her true genius.
One idea could be that Karma and Nagisa have to team up for a special mission outside of school. They face a group of powerful opponents and have to rely on each other's unique skills. Karma's bold and aggressive style combined with Nagisa's stealth and surprise attacks would make for an exciting adventure.
One idea could be an OC who has a unique ability related to stealth. For example, they can blend into the shadows like a chameleon. This OC could use this ability to get closer to Koro - sensei without being detected, adding a new dynamic to the assassination attempts. Another plot could involve an OC who is a transfer student from a different school with a completely different teaching style. They struggle to adapt to the chaos of Class 3 - E at first but eventually find their place and contribute to the assassination plans in their own way. Maybe an OC who is a master of disguise and uses it to trick Koro - sensei into various situations where they can try to attack him.
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.