I just finished watching it again. Its awesome. Nick should put up an effort to put Sozin's Comet up in the big screen. The fight scenes, musical score and plot lines were movie epic level!!!
I know its the ending.. but.. you'll never know....
Book one Water - Shows the discovery of the frozen Avatar in the artic waters , the situation of the the Water benders and their nation.
Book two - Earth - Shows the people and the final defeat of the earth kingdoms by the fire nation.
Book three - Fire - Shows the events within the fire nation, its people and the eventual defeat of the Firelord Ozai.
Book Four (how I wish) - Air - Should show what Happens 10 yrs after from the events at the end of book 3. I mean, hey, the rug rats became tweens, Ben (over at CN), also grew up,so why not show Aang restoring the Air benders to balance the four kingdoms ?
Who knows, The story could also be like, Aang discovered that the monks fought the fire benders, sacrificed their lives so that a remnant can escape. and that This group had hidden themselves, away from all know communication, in a hidden Shangrilla like valley. Who know's, Zuko's mother might be found in this village.
I mean the basis for Book Four is for Aang and friends travel along the lands and lost paths looking for this supposed remant or legendary village of airbenders. Azula could be shown escaping from prison and hunting down Katara in her crazy fit madness way, thus becoming the new central antagonist of the show.
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