I've been reading this lately, got my sister hooked on it too!
Borderlines by:
scuttlebuttincx Pairing: Sokka/Zuko
Author's summary: Three years after the war, the work still isn't quite done and the Gaang is scattered across the continents in their efforts to help the world recover. When Aang and Katara pay visit to the Fire Nation where Zuko is Fire Lord and Sokka acts as Ambassador for the Water Tribe, sparks fly between the siblings over Sokka's life choices.
There are 11 parts/chapters up right now and it's a WIP. I've linked to the first chapter, the rest are
here, just scroll all the way down.
This is the first plot long-and-involved story I've read for Zukka. Might possibly be one of the only ones that exists, even. Takes place when the Gaang is older, around 19 I think. Zuko is Fire Lord and Sokka is the Water Tribe Ambassador staying the palace with Zuko.
I love the tension between the two, nothing happens too fast and everything is so well thought out and in character which is a major plus for me. Once I started reading this, I couldn't stop :D