Perhaps in the fanfiction, Koro - sensei himself time - travels to a point before he became the target of assassination. He could then try to change the circumstances that led to his situation, which would of course have a huge impact on the students and their relationships with him. It would be interesting to see how the students would react to a Koro - sensei who has different motives and knowledge due to time travel.
One possible plot could be a student from the future traveling back to the time of the Assassination Classroom. They might use their knowledge of the future events in the classroom to change certain outcomes. For example, they could try to prevent some of the students from getting into dangerous situations during their attempts to assassinate Koro - sensei.
It could be Nagisa. Since he is one of the central characters in the original series, he would make a great time - traveling protagonist. He could travel back in time to gain more skills or knowledge to better assassinate Koro - sensei, or he might go forward in time to see the consequences of their actions in the classroom.
The plot could center around Nagisa's relationships with the female students in the class. Since she has now become a girl, she might form a closer bond with them. They could start a girl - only club or group within the class where they share secrets and train together. However, this could also lead to some jealousy or conflict from the male students. Nagisa would then have to navigate through these relationship issues while still focusing on the main goal of assassinating Korosensei.
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 possible plot could be that the students from Assassination Classroom are sent on a mission to the world of KHR. They could end up having to help the Guardians in a battle against a new and powerful enemy. Tsuna and his friends would be initially suspicious of these strange new allies but soon realize their skills and unique abilities can be of great help.
Well, a possible plot could be that Nagisa, as a girl, discovers new powers or abilities that are related to her femininity. Maybe she has a special intuition that helps the class in their assassination attempts. The story could also focus on her love life. She might develop feelings for a new character or perhaps Karma in a different way than in the original story. There could be a lot of drama as she tries to balance her feelings with her duties as an assassin in training.
Well, it could be about the students' lives seven years after the events in the original 'Assassination Classroom'. Maybe they've all grown up and are using the skills and lessons they learned in different ways. For example, some might be using their assassination skills in a more legal and ethical way like in the military or security forces.
One possible plot could be that Irina and Karasuma are forced to work on a top - secret mission together outside of the classroom assassination context. Maybe they have to infiltrate an enemy organization where Irina uses her charm skills and Karasuma his combat and strategic abilities. They start off bickering as usual but gradually develop a deeper understanding and respect for each other during the course of the mission.
One possible plot could be a story about Nagisa and Kayano working together on a special project for the class. They might be assigned to plan a surprise event for Koro - sensei. Along the way, they discover new things about each other. Nagisa could learn more about Kayano's acting skills and how she uses them not just on stage but also in her daily interactions. Kayano could see Nagisa's determination and kindness up close, which deepens their friendship.
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.
One characteristic is the exploration of the unique characters. Fanfictions often dig deeper into the backstories and personalities of the students and the teacher in 'Assassination Classroom'. For example, they might expand on how Nagisa came to be so calm and observant. Another aspect is the different plot twists. Some fanfictions create alternate scenarios where the assassination attempts take on new forms or have different outcomes.