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.
All Might and Koro - sensei would be a really unique pairing. All Might is the symbol of peace in the Bnha world, while Koro - sensei is the unkillable teacher in the Assassination Classroom. They both have a strong sense of responsibility towards their students. All Might could be intrigued by Koro - sensei's unique situation and abilities, and Koro - sensei could learn from All Might's heroic ideals. Their combined presence would surely have a huge impact on the students from both universes in this fanfiction.