How about one singular timeloop...that occurred because the heroes lost.
It's something that's been nibbling in my head for a while.
So, final Season, Zuko finally joins the Gaang, Aang faces the Father Lord...and looses. Sokka's airship slice doesn't work out, the razing of the earth kingdom goes as planned. The Avatar is reborn in the water tribes, Sokka, Toph and Suki die on the airships, I forget where Katara was.
And Zuko is captured and locked away in a deep hole somewhere.
Eventually, too soon perhaps, the Avatar dies, and is born again in the Earth Kingdom.
Water benders stop being born.
You see where this is going, I assume?
Zuko manages to outlive his father in his dank cell, they don't bother chaining him up anymore, he is a withered, boney, weak old man.
When the time for the Fire Nation Avatar comes again, the Avatar spirit hesitates.
It rages.
No elements are balance too. But...it doesn't want to die, it doesn't want the spirit realm to be sealed off. And it hates the fire nation. But the part of it that was Aang doesn't
The last fully realized Avatar was Aang, the last time the world had a hope for balance. Zuko is the last remaining person it knew in friendship. It's last link to the past.
Chibi!Zuko, with voices in his head, whispering rage and longing friendship, and old distrust and wariness.
He wants to be a great fire bender so his father will love him, and the voices help, but sometimes the angry voice...sometimes that one makes things go wrong.
He's supposed to be a fire bender, not...Not something his father hates. And he's not supposed to bleed from the ears and eyes when he bends--it must be because he's bad. He fears his bending a little.
Where this goes from here, I don't know.