Yes it's unique. Yes I would like to see a GTA SA 2. But no, I wouldn't like to see a sequel. GTA IV's universe is one of the best I've ever seen in a game, I kinda' wish it would stay that way for 2 or 3 games...
I'm over-optimistic I know.
As for a remake, I see a lot of people who keep shouting at the producers to remake game X (mostly Need For Speed), and all I have to say is this:
Learn to let go of the past, GTA SA was great back then, times change.