I don't know -- I'm not really comfortable with the idea of the main villain being someone not human. There's already a strong tendency to demonize our enemies, see them as something other than human: monsters, animals, evil incarnate. For me, one of the big strengths of the first series was how it did not take the humanity away from its villains. Zhao, Azula, Ozai - they did terrible things, but they were still human.

I could see Amon making a deal with a malevolent spirit -- perhaps even sharing his body for the sake of getting power -- but I would still want Amon the human being to be the driving force, the one making the decisions, the one choosing to do cruel things.