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.
Definitely Nagisa. In the crossover, he could be the one who has the hardest time adapting to the new world of KHR. His assassin skills would be put to the test in a different way compared to Assassination Classroom. He might form an interesting friendship with Tsuna, as both are somewhat timid at first but have great potential within them.
Another storyline could involve a character from Assassination Classroom being sent to the KHR world. Nagisa, with his assassin skills, could get involved in the mafia - like affairs of the KHR universe. He might use his stealth and combat abilities to help out the good guys in that world, or perhaps he gets caught in the middle of a power struggle between different mafia families.
First, understand the personalities of the characters from both KHR and Assassination Classroom. For example, Reborn from KHR is a strict and somewhat mysterious tutor, while Koro - sensei is a fun - loving but deadly octopus - teacher. When creating an interaction, think about how their different traits would clash or complement each other. Maybe Reborn would be intrigued by Koro - sensei's unique abilities and try to find a way to use them for his own training methods.
Well, it could be really exciting. Imagine Ruby meeting Nagisa. Ruby's scythe - wielding skills combined with Nagisa's assassination techniques would make for some amazing fight scenes. And Weiss could interact with Karma, and there would be a clash of their personalities. Plus, the Grimm from RWBY would be a whole new level of challenge for the 'Assassination Classroom' characters.
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.
I think Ruby and Nagisa would be a great pairing. Ruby is all about being a hero and has her own sense of justice, while Nagisa has been trained to be an assassin but also has a good heart. Their different outlooks could lead to some really interesting conversations and adventures.