i myself (the author) have often wonder, what would have happened if Zuko did not join the gang. well, Ang would not have learned fire bending and properly been killed by the fire lord.
so that's what happen, well in this version of the story. The fire kingdom has taken over, with the new king, Zuko, but he is not the same person as you and (mostly) i love. He is now cocky, selfish, and ignorant. He has ordered that all avatars be killed. the got the ones that were born in the water and earth kingdom, but not the one living in there very own kingdom. They missed a 16 year old girl with the name of Kasa.
the most wonderful story begins yet again but with a frightful twist.