There’s no confirmation on this, so take it with a grain of salt, and also beware that there are SPOILERS ahead if you clicky this link. If that wasn’t spoiler-y enough for you, there are descriptions (also unconfirmed) here.
Personally, I believe that the best achievements in GTA games aren’t listed, because they can’t be predicted. There should be an achievement where you upload a replay of something utterly insane that just happened, and if it meets their standards you get the achievement. Then they could pack it up and send it back out as free content to put on the ingame TVs you walk past. How am I not a game developer?