After a rewatch of Siege of the North, a particular interchange stood out to me during the scene where Aang speaks to Koh the Face-Stealer. Once Aang has received the information he came for, he tells Koh that he has to leave, to which Koh responds, "We'll meet again". However, as far as I know, Aang never met Koh again. So I began to wonder if there is some untold story where they did have another encounter, perhaps sometime in Aang's adulthood? But it seemed like too much of a significant departing sentence for the story to be left untold. Ever since I first saw this episode, Koh has struck me as extremely viable for a prospective villain. He's sinister, manipulative, powerful, and down right creepy. Not to mention that he's the only spirit we ever meet in the series who truly seems evil (I know that assumption is debatable, but that is what he comes across as); Hei Bai turned out to be good, he was just ticked off about his forest, and Wan Shi Tong did end up being an adversary, but he was only concerned with his precious library. So, if there's going to be a future altercation between Aang and Koh, wouldn't it be a tale worth telling? The opportunity to use Koh for a future plot twist just seems to good to pass up. Before I continue, I'll go ahead and acknowledge that on Koh's page here on the wiki, it says that they did meet again when Aang was in the Spirit World after Azula shot him in Ba Sing Se, and that he tried to steal his face before he could meet up with his past lives. BUT this statement is not cited. Also, I watched the "Escape from the Spirit World" video, and Koh did not make an appearance even though Yue said he would. So this information could be false, and if anyone has any knowledge about this that they can prove and back up with official sources, please let me know! It might kill my theory, but it's more important to have the truth rather than hopeful musings.
Moving on. I considered the possibility of Koh reappearing in a future comic that features Aang, but so far there is nothing being released that contains any Spirit World connotations. Then I remembered; Season 2 of LoK will be entitled "Spirits", which makes it riddled with possibilities for scenarios involving Koh. We've also been told that this season will contain lots of flashbacks. Is it plausible that we will see a flashback of Aang and his second meeting with Koh? Or... might Korra meet Koh herself? When Koh mentioned meeting Aang again, he could have actually been refering to the Avatar spirit itself, and not specifically Aang. So here's my concluding theory: Korra will cross paths with Koh sometime during season 2, therefore fulfilling Koh's prophecy. Can you imagine the ways Koh would try to elicit an expression of emotion on Korra's face, so that he can in turn steal it? And now imagine her actually attempting to hold a straight face while being provoked by a vindictive spirit. I don't know about everyone else, but I'm thinking that that would be more of a struggle for Korra than it was for Aang.
So that's pretty much it. Any thoughts? Can anyone think of a reason why the creators might put this somewhere in the next season? I have an idea, but I want to see if there are others interested in this theory first. Oh, and if anyone can definitely prove that Aang and Koh met twice, make it known! However, even if they did, it's still possible that Koh could reappear in LoK.