Two theories on who Amon might be:
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Ozai. Yeah, I know he'd be ancient, but think about it. His bending was taken away from him, so it makes sense he would want revenge by taking everyone else's bending away.
Also, Tarrlok mentioned one of Aang's old adversaries, so I wonder if it could be him back for revenge. Adult Aang was fighting someone during the flashback. Maybe it was him.
Both theories have connections with the old characters and would explain the flashback sequence.
Where sky and water meet,
Where the waves grow sweet,
Doubt not, Reepicheep,
To find all you seek,
There is the utter east.