I haven't got the spoons for writing anything long, so I'm just throwing my ideas out here in little snippets. This little snippet turned out a bit long, but...
This is super dark. I've tagged all of the dark stuff, and if I've missed a tw, please tell me. I don't think I did. There are no graphic descriptions.
ATLA AU: Aang defeats Ozai. Aang loses everyone.
Aang has just defeated Ozai. He looks out to the horizon, and sees a number of airships crashing down. He mourns the possible loss of life (even though he can see many crews huddling on the tops of the airships, surely there were some who didn't make it) but ultimately, he's at peace with it.
He sees one particular airship go down rough. Well, he thinks. Time to go rescue people.
He secures Ozai to the rock and even though the man is a monster, he promises to come back and that the former Fire Lord will be treated humanely.
There are bodies in the wreck, and he offers some words to Agni on their behalf, because that's who he is, and he remembers that Fire folk need to be guided on their way to the Sun. He remembers the words from a hundred years ago like yesterday, because for him, it was. Well, yesterday and a year.
But when he finds three bodies, huddled close together with a burst steam pipe near them and the room slowly filling with water, Aang finds that he doesn't have any words at all.