Korra is kind of... dumb.
Wild speculation:
Tenzin is Amon.
...assuming they actually want to surprise us when the mask finally comes off, it's the only choice that makes sense. Tarrlok seems kind of too obvious; Mr. Sato is a candidate, but at this point it lacks any dramatic value.
Even wilder speculation:
It's all a VR simulation played by an Avatar even further in the future to... okay, I got nothing past that, I didn't think this out, I told you it was wild.
Tenzin is Amon.
...assuming they actually want to surprise us when the mask finally comes off, it's the only choice that makes sense. Tarrlok seems kind of too obvious; Mr. Sato is a candidate, but at this point it lacks any dramatic value.
Even wilder speculation:
It's all a VR simulation played by an Avatar even further in the future to... okay, I got nothing past that, I didn't think this out, I told you it was wild.