Reborn would be an extremely interesting character to focus on. His no - nonsense attitude and his ability to train Tsuna could be applied to the students of Assassination Classroom. He would be intrigued by Koro - sensei and might come up with some wild plans to either help or hinder the students' attempts to assassinate him. He could also be a source of conflict as he might try to impose his own methods on the students, which they may or may not like.
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.
Naruto himself would be really interesting. His never - give - up attitude could inspire the students in the Assassination Classroom. He could also learn from their unique skills and knowledge. His journey in this new world would be full of surprises as he tries to adapt to their assassination - based curriculum.
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.
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.
Midoriya and Nagisa would be an interesting pairing. Midoriya is all about becoming a great hero and has a powerful Quirk that he's constantly learning to control. Nagisa, on the other hand, is a skilled assassin - in - training with great stealth. They could learn a lot from each other. Midoriya could teach Nagisa about the importance of having a big dream and standing up for what's right in a more heroic sense, while Nagisa could show Midoriya how to be more stealthy and observant.
I think Rock Lee and Gokudera would be a fun pairing. Rock Lee's extreme dedication to training and his unique fighting style would surely amaze Gokudera. Gokudera, in turn, could introduce Rock Lee to the power of the Dying Will Flame and how it can enhance combat abilities. Also, Kakashi and Reborn would be an interesting match. Kakashi's vast experience and intelligence combined with Reborn's shrewdness and unique training methods would make for some great interactions.
Raphael could be really interesting. In the 2003 version, he is hot - tempered and tough. In the 2012 version, he also has a strong personality. Seeing how he reacts to his counterpart and how they might either clash or bond could be a great focus. Maybe they have a friendly rivalry as they compare their fighting skills.
Ross could be really interesting. Since he's an archaeologist, in a crossover with a show like 'Indiana Jones' (in a fanfiction sense), he could go on an adventure with Indiana Jones. Ross would bring his academic knowledge while Indy would have the practical experience. Their interactions would be filled with Ross trying to over - analyze everything and Indy just wanting to get the job done.
Chiaki Nanami could be an excellent choice. She is the Ultimate Gamer, so if you cross her with a digital or virtual world, there are so many possibilities. She could use her gaming skills to navigate through a new digital realm, like a Matrix - like world. Her calm and intelligent nature would also make her a great character to lead the group in a crossover situation. She could figure out the rules of the new world quickly and help the others adapt.
Reborn and Erza. Reborn is a strict and highly skilled hitman - turned - tutor, and Erza is a powerful and disciplined mage. They could have some intense training sessions together. Reborn might be impressed by Erza's combat skills and her ability to lead in battle, and Erza could be intrigued by Reborn's unique teaching methods and his vast knowledge of combat and strategy.
Definitely Damon Salvatore. He's the bad - boy vampire with a heart of gold (sometimes). In a crossover, he'd be a wild card. He has his own sense of justice and loyalty, but also a devil - may - care attitude. He could get into all sorts of trouble in the world of the Originals. His sarcastic remarks and his ability to charm his way out of (or into) situations would make for some great scenes. He might also form an unlikely friendship or rivalry with one of the Originals, like Kol perhaps. His backstory and his experiences in 'The Vampire Diaries' would contrast and collide in interesting ways with the Originals' world.