I don't own Avatar, if I did Sokka would have a lot more serious moments, etc. etc.
General rule of thumb for this fic: put yourself in the frame of mind that Avatar was something of an RPG in which you could choose the dialogue, if not the overall story, and these are the possible ending scenarios for Sokka, depending on a host of invisible factors like hidden plot flags or an approval system within the party that the player could manipulate. If a epilogue doesn't quite play along with the canon... well, some players play differently and like different characters more than others.
Why? Because I find it fun that Sokka could most plausibly do the greatest number of things after the series ended.
This entire fic has been written, posting it in my peculiar style of a micro a day.
Epilogue
When Sokka is closest to Aang, it is because it is the closest to being a big brother to a fellow male he'll ever get. Camaraderie, loyalty, yes, those are all there, but it's the ability to mentor that sets it in motion.
Because, for all that Aang is a hundred and what-not years old (technically), and Sokka never saw a girl near his own age who wasn't his sister until they left, Sokka knows stuff. Guy stuff, life stuff, funny stuff, or just stuff. And he's happy enough to share, and somehow discrete enough not to blab.
So when Aang has awkward questions about girls, Sokka shares timeless wisdom he heard third-hand from the tribe's real men. When Aang is a bit too trusting, a bit too quick to fall for a scam or story of the world, Sokka's there to fish him out. And when Aang hit puberty… well, Sokka doesn't laugh too much when Aang's voice cracks.
And when Aang is faced with whether to use his powers for the war or not, Sokka is there to help him remember what is really important. When Aang can only face what he can do, he relies on Sokka to come up with the plan. And when Aang holds the weight of the world on his soldiers, Sokka's right there to help him bear it. There remains almost nothing Sokka wouldn't do for his friend, even give away his sister's hand in marriage, and there's no limit to Aang's gratitude, and reciprocity, to that feeling.
Why would that trust, that brotherhood, change one iota after the war?
After the war, Sokka continued his travels with the Avatar, helping to restore balance to the world. While his name would forever be overshadowed by Avatar Aang's, as all other names from the period tended to be, his roles as an ever present companion, confident and advisor would not. Aang and Sokka spent much time over the years traveling together, whether mediating international disputes, arguing the relative merits of a vegetarian versus meat diet, or simply in close proximity as brothers-in-law, though both men would consider their status as brothers to long precede that.