Koh is certainly high on my list, though "can't steal" is a variant that I don't think I'd seen before, which I like--not that the mask would _stop_ Koh if (it/he? don't remember) wanted, but it would give Koh a plausible reason to skip the face-stealing attempts and focus on fucking with the Avatar.

The chi-blocking theory is interesting but I'm dubious, because a temporary deprivation that's billed to be permanent seems a really big risk to me. Also I like the implications of it being energy-bending better. => But I guess we'll see!