Honestly, I don't get why fandom makes such a huge deal over Koh. The more people build him up into this ultra-important arch-nemesis that we HAVE to see again, the more turned off I am by the hype. Maybe there's another story to be told there, and I can understand why people are curious about the gaps in what we've been shown, but that doesn't necessarily mean the character is worth building a whole season around. I mean, ooh, he steals faces. Weird, but not earth-shattering. And Aang was able to beat him just by not changing facial expression -- which doesn't really make Koh seem like that massive a threat.