Disclaimer: I don't own Avatar. This story was written for fun, not profit.
There are times when Aang scares him.
Okay, fine, Aang's always a little strange, like he's seeing the world underwater or something. Sokka isn't sure if that's because of the Avatar thing or the frozen-in-a-glacier thing or just some innate Aangness - but he's most comfortable when he thinks it's the latter, and far be it for him to take the difficult approach. There will be time for that later.
And there will be a later, no matter how hard he tries to forget it, no matter how much Katara fusses at him for being a boy and tries to fill this great big mother-shaped hole in their lives. Most of the time he tries to ignore the sense of foreboding, or he just blames it on Aang.
(And he wonders if anyone can bend foreboding so it hits at the worst possible moments, like when he doesn't want to dream about flames and red skies and dead gods and Yue - which is always.)
He thinks, on some really basic ohshit level, that it's going to be bad. Then he thinks that he has a talent for understatement and no skill whatsoever with words, because "bad" doesn't quite encompass the feeling that the air's too thick and time's too short and he'll fail them.
Yeah, he's afraid. There's this annoying corner of his head that's afraid all the time -- and more afraid for Aang and Katara and even Toph than for himself, which is a whole new level of scary. They're his family and friends and his own personal tribe (even if he's so bad with words he can't quite put it that way) and he knows that him-or-them isn't really a choice at all.
This all leads back to the foreboding thing, which feels like the Fire Nation fleet breathing down his neck and sounds like Aang making big metal ships into so much scrap and looks like Katara fighting. (Which she shouldn't be doing; he's the protector here, she's his know-it-all kid sister and he can't protect her.) And he shoves it into tight little corners of his mind and fends it off as best he can, because there's nothing else he can do.
Sokka knows he's not the smartest or funniest or best, so he grins and gripes and stumbles because this might be the last time he ever does any of these things. That scares him too - because when Aang's more spirit than dumb kid sorta-brother, he thinks he can see the way they're all going. He doesn't know if they'll win or lose, but he knows that sooner or later it'll be bad in every sense of the word.
He hopes it's later, when Katara's old enough to scare granddaughters and Toph's teaching scary Earthbending warriors and Aang's delivering cryptic pronouncements from atop a bison somewhere, because it's not their job.
Except it is. He thinks maybe it's always been.
And that scares him more than any power-hungry Fire Lord ever could.