One possible fanfiction could be about Nagisa, as a girl, having to deal with the unique challenges in the Assassination Classroom. Maybe she uses her newfound femininity to her advantage in different assassination attempts. For example, she could play on the stereotypes that others might have of a girl being less of a threat, only to surprise her opponents.
One possible fanfiction could be about female Karma's journey in the Assassination Classroom. Maybe she uses her intelligence and unique skills to outwit the other students in various assassination attempts. For example, she could come up with creative plans to get close to Koro - sensei while still maintaining her cool and mysterious demeanor.
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.
She could have a secret power that only she discovers over time. For instance, the power to read Koro - sensei's thoughts briefly when she makes direct eye contact with him. This would give her an edge in the assassination attempts. She could then use this power to set up elaborate traps based on what she learns from his thoughts.
Well, you could write a fanfiction where the students develop new and unique assassination techniques. It could also be interesting to write about a time - travel plot, where a student goes back in time to change something that would affect the whole assassination classroom scenario. Or perhaps a story that focuses on the relationship between Koro - sensei and a particular student, like Nagisa, and how it develops in unexpected ways.
One popular theme is the relationship between the students and Koro - sensei. Many fanfictions explore how their bond grows and changes over time. Another common theme is alternate universes. For example, an AU where the assassination plot never happened, or where the characters are in a different setting like a modern - day high school without the assassination mission. There are also fanfictions that focus on the backstories of individual characters, like Nagisa's life before coming to the classroom.