When writing the fanfiction, focus on character development. If Naruto is in the Assassination Classroom, how would he react to Koro - sensei's teaching methods? How would the students in Class 3 - E view Naruto's ninja skills? Also, create a good plot structure. Maybe start with a small incident that leads to a bigger conflict, like a misunderstanding between Naruto and some of the students that escalates into a full - blown battle scenario where they have to work together in the end.
One good example could be a fanfiction where Naruto characters are transferred into the Assassination Classroom world. They could use their unique ninja skills in the context of trying to assassinate Koro - sensei. For instance, Naruto might use his shadow clone jutsu to create distractions while the others plan their attacks.
Maybe some of the students from the Assassination Classroom end up in the Naruto world. They would be in for a shock with all the chakra and ninjutsu. They could learn from the ninja and apply their own assassination skills in a new context, like trying to 'assassinate' a powerful ninja for training purposes.
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.