This is a good idea, but it needs some overall work.
When in complete rage (a.k.a. avatar state), his eyes and all of his arrows are white. Make him angry and not happy because he is in complete rage (i.e. make the mouth upside down and thinner, and add frowning eyebrows). Make it when he is in Avatar state, add some glowing around the eyes.
The chest needs some work not some random shape. Suggestion: Use the bottom part of the tunic and
get paint.net to make the torso better, and make it completely even for better looks. If you have mastered Decal making (which I see you have not) then you could make it uneven and make his tunic waving like in the wind.
You have a good idea here and I suggest you or someone else continue this, but this is a lot of edits.
And btw, it would be epic to make other characters like Sokka, Toff, Katara, Zuko, Azula, Boomy, etc...
Btw, watch the Avatar series. It's epic. Epicly awesome if I'm not mistaken.