Honestly, I wouldn't mind if they took another year, maybe more, to make the next game, because they'd be building off of GTA4's engine, they could expand into so many new territories. And GTA has established itself time and time again that it is the dominant free roam game, I don't think they need to worry too much about someone else stepping in.
Of course, I just got the game a few months ago because I waited that long to upgrade to PS3, so maybe thats why I don't mind the wait. In the meantime, I've barely started the main missions, considering buying EFLC, probably will get Just Cause 2 eventually, and maybe Red Dead Redemption. And of course, my beloved Driver San Fransisco when that comes out (that'll be a first day buy if I can help it), even if they removed on foot (stupid decision, but the driving I think will still be top notch)