I really want to throw Nagisa at Xiao more! He IS a squishy human being who is probably susceptible to the effects of Xiao's karmic debt, but a) he also finds communicating with people hard (for different reasons than Xiao but still) and b) would not be bothered by Xiao's oddities like teleporting out of conversations because [gestures at Ensemble Stars!!] look at this canon full of weirdos, and c) would encourage Xiao to dance and enjoy his life while he can. I think there could be some interesting CR there.

Xiao is always welcome to swoop in and save Nagisa whenever he accidentally wanders into a quest, too.