Timeline? What timeline? We don't need no stinking timeline.
Bear with me? The Gaang will make its appearance in all its diverse glory, I swear. This is an idea.
The Fire Lord turned to face the Avatar, who was looking back with that expression that made the Lord of Flame and Fury want to grind his teeth in a most unbecoming and unkingly fashion, the sardonically mournful expression that said "Old friend, you are being foolish and I am tolerating you only because I have this inexplicable fondness for idiots," the expression that made him feel stupid and vaguely guilty at the same time.
But he was a grown man, and Lord of the Fire Nation, and he would be damned before he let that bumbling buffoon (because the man was a buffoon - what self respecting Master of the Elements, however learned and enlightened, spent his days earning pennies as a street performer, and a mediocre one, at that? He didn't even use his bending! It was all sleight of hand. He wasn't even very good at it) before he let the mere facial expression of a buffoon sway him from realizing his most treasured dream. No. The Fire Nation would be great, and there would be peace, no matter if he had to sell his soul to achieve it. And anyone else's soul, too, if it became necessary.
The Avatar kept staring at him, sorrowful and exasperated and not a trace of the fear that would have permitted the Fire Lord to despise him. And so the Fire Lord spoke. "I cannot permit emotion to stand in my way. The Fire Nation needs this. The world needs this. This is a necessary and right thing to do." And it still sounded too much like he was trying to reassure himself.
"The world was doing all right on its own, if you ask me."
"Oh? And what about the Water Tribes, where girls are married off to old men for a couple of barrels of whaletortoise fat? Or in the Earth Kingdom, where small children are sent into the mines, not even for the blessed gold but for worthless gems? Or how about in the Air Tribes, where there is no such thing as family, because the boys are raised to be monks as soon as they are weaned, and the women flit around with their damned herds, drink their damned milk and only value their daughters? There is so much injustice in the world. I will change that. You could have helped me. Think of the honor and the glory you could have achieved."
The Avatar looked up, his expression hardening into the one were the Fire Lord had a hard time describing him as a buffoon even in jest, even in the quiet confines of his own mind. "Domination by the Fire will not right the wrongs in the world, old friend. All you will do is pile new evils atop old, except these evils will be in your name."
His hand tightened on the knife. "So be it. Maybe we will meet again. Goodbye, Roku."
Fire Lord Sozin, Lord of Flame and Fury, Keeper of the Golden Keys of Agni, Firebending Master and best friend of Avatar Roku set the body down gently, respectfully. Then he stepped back from where Roku's blood still ran warm on the altar. He washed his hands in a bucket that he'd had brought and placed by the door. He went outside, something burning in his veins. He looked up into the bright noon sun, something thrumming through his stomach and up and down his fingertips. He roared in what was certainly not grief and turned back to face the bloody altar through the open door of the temple and shot a bolt of sizzling blue flame from his palms. The stone split down the middle and Roku's body started to burn, unnaturally and far too quickly. In moments the body was gone, and so was what whatever Sozin had been feeling. He started to shake. He hugged himself as best he could through his armor.
The seer had said that it took time for the divine fire to move once it had been directed; it would come in the number of fingers on a witch's two hands. Twelve years. He sat down to wait for his comet, the thing that would give him enough power to save the world. The thing that would make this seeming sin an act of grace.