My theory is rather simple.
There is only two real answers to "Who is Amon?"
A). Very easily, he is someone brand new we have no prior connection with.
B). He gave us a hint during the 3rd episode "The Revelation". Before he makes his revelation of what he can do to benders. He tells us a story from his childhood. About how his village was attacked by firebenders...leaving his face scarred among other things.
It is my opinion that the only character that Amon could be from A:TLA...is Lee from the Earth episode #7 "Zuko Alone". In where Lee is a small boy (younger than Aang was), and whose family and village as a whole was being bullied by a group of rougue soldiers. Zuko helps this boy and his family defend themselves. When it is revealed Zuko is a firebender, Lee and his family reject him outright. Lee is especially suspicious, as his demeanor towards Zuko changes drastically. It is clear he hates firebenders, physically telling Zuko he hates him...even after Zuko just saved his life.
Lee demonstartes every trait a villain could have as a young boy. Stealing anything he can get his hands on whether that is food...or broadswords. He isn't afraid to confront those older and more powerful than he. While friendly as a boy, his distrust of firebenders, especially the way he reacted to Zuko, could very easily be a life long trait, and eventual obsession.
We do not know what happens to Lee or his family after Zuko willingly leaves the Earth village. It is very possible, by foresaking Zuko's kindness, his village and his family could have come to harm from other firebenders...who were not as kind as Zuko.
I am convinced this young boy Lee who would be approximately 79-80 years old in the new series makes the most sense, if the character is from the original series. We do not know what Amon's face is like, nor how old he is. If age is a factor, and I still believe Lee could very well be 80 years old. It may also be possible that Amon could be Lee's son.
Edited on 04/29/2012 1:22pm