Sunbeams
Ursa doesn't find out about Ozai's plan. Ozai goes through with it. Zuko is dead, Azulon is dead soon after, Ozai is Fire Lord with Ursa at his side.
Agni is not so happy to find his champion killed at age eleven.
So instead, he sends the boy off on a spirit quest to prove himself worthy of life once more.
But Raava's champion is not in contact, Oma's is too much, Shu's is in a bad position, Athe's is too, as is La's...
So, Agni sends the eleven-year-old spirit of Prince Zuko to the champion of Tui, Princess Yue, the moment that Avatar Aang reawakens. The problem? Zuko's a SPIRIT. And a child. And the only one who can see him is Yue.
Basically a "Ozai actually does kill Zuko but Agni is having none of it so when Aang returns he uses the boost in spirit magic to send Zuko's spirit to Yue. Yue takes this eleven-year-old-boy-that-only-she-can-see and decides 'yes this is my new emotional support firebender' and the two get closer and closer until the Gaang finally arrives in the Northern Water Tribe and shit hits the fan" AU. So, all-in-all, a Yue-Zuko-bonding adventure featuring confused bb! Zuko and will-kill-anyone-and-everyone-who-hurts-her-little-fire-ghost-boy! Yue