A great plot could involve a time - travel element. Koro - sensei accidentally sends the students back in time to the era of KHR. The students get involved with the Vongola family's affairs. Some of the students might form bonds with the younger versions of the Guardians. They could use their knowledge from the future (like assassination techniques and scientific knowledge) to help the Vongola in a difficult situation, while also trying to find a way back to their own time.
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.
Maybe there's a powerful dark magic that is threatening both the KHR world and the Fairy Tail world. So, the Vongola Family decides to team up with Fairy Tail. Tsuna could bond with Natsu over their determination to protect their friends and families. The two groups would combine their unique powers. Like, Reborn's assassination skills along with Erza's swordsmanship could create a really cool battle strategy against the common enemy.
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.
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.
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.
A really interesting plot could be where the reader is from a different time period and gets transported into the 'Assassination Classroom' world. They then have to adapt to the strange situation of trying to assassinate a teacher while also dealing with the modern - day school life. This could lead to a lot of humorous and heartwarming moments as they interact with the characters.
One interesting plot could be a gamer who gets transported into the Assassination Classroom world and uses their gaming skills to outwit the students in assassination attempts. For example, they might use strategic thinking from real - time strategy games.
Here's a plot. Nagisa decides to take a break from the assassination classroom and goes on a solo journey. During this journey, she meets new people who teach her different skills and ways of thinking. When she returns to the class, she has a new perspective and is able to contribute in ways she hadn't before. Another idea is that Nagisa gets involved in a mystery within the school that has nothing to do with assassinations at first, but as she digs deeper, she realizes it's connected to a larger threat that the class will eventually have to face. And then there could be a plot where Nagisa has to compete in a school - wide competition against other students, and she has to use her assassin skills in a non - lethal way to win.
One idea could be that the students from Bnha are sent to study in the Assassination Classroom for a special mission. They have to learn assassination skills while also trying to incorporate their own Quirks into the training. Another plot could involve a villain who has abilities similar to Koro - sensei from Assassination Classroom threatening the Bnha world, and the heroes from both universes have to team up to stop him.