Kayano would be completely devastated. Since she had a special bond with Nagisa, she might become withdrawn and filled with rage, vowing to take revenge on whoever was responsible.
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.
A 'classroom assassination nagisa fanfic' is likely a piece of fan - fiction centered around a character named Nagisa in a classroom. The 'assassination' part could imply a lot of things. It could be a metaphorical assassination of Nagisa's character, like an attempt to ruin his reputation in the classroom story. Or it could be a more literal, action - filled plot where someone is out to kill Nagisa in the classroom setting. There could be a whole backstory about why this is happening, such as rivalries, secrets, or power plays within the classroom's fictional world.
One possible fanfic could be about Nagisa, as a girl, having unique interactions with the other students in Class 3 - E. Maybe she forms a special bond with Karma that is different from the original story due to her changed gender. She could use her skills in a new way, for example, her stealth might be enhanced by her newfound femininity, as she could blend in more easily in certain situations. Another aspect could be how she deals with the challenges of being a girl assassin in training, facing different expectations from her classmates and teacher.
In the 'Assassination Classroom' series, Nagisa is male. In fanfics, while some might play with the concept in creative ways like gender - bender scenarios for the sake of unique storylines, traditionally Nagisa is a male character.