(Here's a link to learn more about that episode: The Siege of the North, Part 2.)
Now, back to Koh. One would say that Koh didn't have a face or maybe that he had stolen numerous faces to the point where his original face had been forgotten. This is where we can spot some connections. Amon wears a mask, allowing his face and who he truly is to be unknown. Since Amon's face is unseen, we can say that maybe he doesn't exactly have a face due to being Koh or being controlled by Koh.
Also, Amon is able to supposedly take people's bending away. How can Amon learn such a thing? Aang was able to learn how to energybend, in which allowed him the power to take away people's bending, from the lion turtle. Only the Avatar can communicate with these spiritual beings. That is why I can conclude that maybe Koh was able to teach Amon how to do such an advanced chi blocking move.
Koh also stated to Aang that they'll meet again. Although Koh was talking to Aang at that time, Koh had said "Avatar" and not Aang's name. This could mean that Koh is planning to be involved with later Avatar generations, for example, the new avatar in Legend of Korra!
This is merely a theory that was mentioned during a break between classes. I know it may seem a little too wild and that the facts may not be enough to support this, but it just seemed too interesting!
This theory is a little undeveloped, so if you agree, please leave some feedback in the comments! If you don't agree, say why and maybe add your own theory of who this mysterious being may be!