Long ago, the four nations lived together in harmony. Then everything changed when the Air Nomads attacked. ...

This needs to exist. I'm hoping this post inspires someone to write it O:-)

In terms of background it could work as a theocratic fanatical movement taking charge of the Air Nomads, establishing a deadly powerful air force with large gliders, armored sky bison, and possibly airships.

It seems most effective if they have seized Omashu to power the war machine before the story opens, but it would probably be more interesting if Ba Sing Se were still the Earth Kingdom capitol.

An interesting question would be whether to go with Aang running away from the fanatics, or a complete AU with a waterbender avatar after the airbender avatar is killed by the fanatics.

Inspired by Commanding the Tide