Megumi x yn? Honestly, I'm not even sure I've seen that many stories center on that ship specifically, which might be part of the appeal. The dynamic is usually built on a certain tension—he's reserved, a bit closed-off, and you're this variable that forces him out of his shell. I see a lot of 'enemies/rivals to lovers' setups where the yn is maybe a sorcerer from a rival family or a special grade curse that's semi-human, creating that push-pull conflict.
Another common thread is the 'mission gone wrong' trope. They get stuck together on a long-term assignment, forced proximity in some remote village or a cursed domain that isolates them. The slow build of trust and eventual vulnerability feels earned. Also, protective Megumi is huge. He's always got his shikigami, but seeing him genuinely worried for someone else's safety, maybe after the yn gets hurt, unlocks a different side of him. The found family angle with Tsumiki sometimes gets woven in too, which adds a sweet, domestic layer.
I think writers latch onto his quiet intensity because it leaves so much room for subtext. You can fill in all the things he's not saying.