So this was more or less inevitable
1.
Though they are brother and sister they are as different as night and day, moon and ocean, and they argue constantly. Their bickering is like a stream that chatters continually from day to day- always shallow, never raging, flowing from a source that no-one can find nor remember. When the war comes it simply gives them something else to argue about. The sister fights on one front and her brother on another. Help is not asked for and never given, and in the tides of war the siblings do not see each other for many years. So it surprises everyone but themselves that when the proud Northern brother loses his white-haired princess, his Southern sister holds him while he cries under a bright, unblemished moon.
2.
The Earth Rumble has its own culture and heroes. It's an undisputed fact that The Boulder is the greatest champion of his generation, or that the Blind Bandit will be the hero of the next. The champion of the first Earth Rumble was the Man Mountain, a large but unshakeable fighter who simply took all that his opponents could throw at him before striking with the sudden savagery of an earthquake. Few people talk about him, and even fewer can recollect what he looked like, but Toph remembers her first visit to the Earth Rumble tournament and the kind, reassuring presence beside her that explained the few rules and cheered to break the stunned silence that followed her first ever win. Telling the story to Sokka many years later, Toph smiles and pats the earth beside her. "He was solid, y'know? Like a mountain."
3.
Every Fire Nation child learns three things- first, the Fire Lord is mighty. Second, the Fire Nation is glorious. Thirdly, the world is theirs by right. After one hundred years of nothing but conflict, these things form the backbone of a nation who barely recognizes himself in the small circle of brass that he uses for a mirror- his reflection flickers between despot and conqueror, soldier and citizen. When he kneels before his Fire Lord, he wonders if the hundred years' heat of battle has not tempered him, as Sozin promised, but instead warped him beyond all recognition.
4.
Aang dreams every night. Sometimes they are half -remembered echoes from his hundreds of past lives- a woman with flowing brown hair, an island breaking from the mainland in a hail of lava and rock, halls of red marble and shining gold. Sometimes they are nightmares of the massacre of his people, his mind filling in all that he missed in a tornado of detail so vivid that he wakes up sweating, unable to close his eyes again. Often, however, he dreams of a woman, tall, slender, her head shaved and tattooed in the manner of his people. He knows that he has never met her in all of his many lives, but she is still as familiar to him as his own heartbeat, her presence as natural as breath. In his dreams, they fly together around temples bustling with life and laugh at jokes that no-one has told in a hundred years. The night after he defeats Ozai, she kisses him chastely on the forehead and tells him warmly that as long as he lives and remembers, so will she.
He never finds out her name.
Written in a big rush at about half eleven last night, before I forgot everything :p. The Fire Nation was a pig to write, I'll tell you that.