I think GTA5 will be the game everyone was expecting GTA4 to be. Why? Because GTA4 was the first, or perhaps second, game to use the RAGE engine. They had to build everything from the GROUND UP for it.
Since then, though, GTA4, as well as it's two Episodic sequels (TLADT and TBOGT), as well as Red Dead Redemption, LA Noire, Max Payne 3, Midnight Club: Los Angeles (*cough* *cough*) and probably others as well were ALL built using the RAGE engine.
The conclusion you can make from this: with GTA4, they had to build the entire game from scratch, including a brand new engine. It probably had less features/less depth because it was the first BIG game running on the new engine. Which means, all of the time they had to spend troubleshooting, honing and perfecting the RAGE engine, was time they couldn't spend on doing OTHER things developmentally.
However, fast forward three years to 2011 (when GTA5 is announced). Since GTA4's release, at least seven games (mentioned above) have been built using the engine. They've got models saved on some hard-drives of Wild West style stuff (from Red Dead Redemption) and TWO games to pull Los Angeles style models from for GTA5 (LA Noire and Midnight Club: LA, anybody?) all built using the RAGE ENGINE! This is all stuff they ALREADY HAVE DESIGNED AND SAVED.
They won't just take it exactly and copy/paste it, sure, probably not, but I don't necessarily think the fact they are doing all of these Los Angeles based games around the same time is ANY surprise at all. In fact, it's a good thing. They will end up having to spend less time building the city and can spend more time on fleshing the game out, and actually making it fun. By this time, they've mastered the engine that they created for GTA4. They've done the hard work.
GTA5 will be the game we all wanted GTA4 to be. Not only do I just know it in my heart, but from a logical perspective, now that the hard work in game development is done -- they have the ability to spread their wings and fly with this title, reminiscent of what VC and SA were like compared to GTA3. Mark my words, this game will be what we've all been waiting seven years for, basically.