"Unexpected adventures in Southern San Andreas..."
Kind of weird to call out the location like that, hints that they're expanding the geography of the game. There's a lot of talk throughout the game of running drugs and weapons over the border with Mexico, it's a big theme early in Trevor's storyline... one of the first missions with Trevor, you're flying a plane and talking about it, and then in other subsequent small missions, Trevor mentions his experience working with the Mexican drug cartels and drug mules.
I suppose "southern San Andreas" could just mean Los Santos, versus BLaine County, but why wouldn't they just say "Los Santos" instead?
I wouldn't think San Fierro because, well, San Francisco is well north of Los Angeles. Las Vegas / Las Venturas? Maybe?
Anyway, I'd really die for some more content in this game. After the horrible mess of GTA Online, I'm more eager to play the SP portion of the game... but I've pretty much done everything that I'd want to consider doing, minus the letter collection and sub missions,w hich I don't have much patience for.