I just completed the GTAV story, but have not tried going online yet.
The thing that impresses me the most about GTAV is the phenomenal level of variety and detail--overall realism--in the game world. There are several recent games that I have not yet played, but in my experience no game comes even close to GTAV's combination of world size and world realism. It is incredibly difficult to find an architectural asset that is used more than once.
I couldn't help but thinking about how much gameplay opportunity there was within such a realistic environment. And although I would love a new environment as much as anyone, it occurred to me that it is not even necessary to construct an entirely new world for a new game. Now please, hear me out, because I am the last person to defend laziness in any way.
First, understand that there is always a limit to what is possible at a given time (and within a given time)--hardware-imposed limits and financial/human resource-imposed limits. Let me propose an alternative to a new world:
What if all of Rockstar's resources were devoted to taking GTAV's world and making each and every interior accessible. Yes, I mean each and every interior of every building. Every store. Every floor of every skyscraper. Every last mansion and trailer. "Enters Everything You Ever Saw". I would really rather have this than a new world with minimally or partially accessible interiors.
The resources that would have been devoted to making an entirely new world from scratch would go to modeling the interiors with the same attention to detail that went into the exterior. To creating thousands of unique assets with which to populate the interiors. And although I consider that to be the greatest improvement because of what it would do for gameplay opportunities, that is by no means all.
There are plenty of open areas with which to add new structures and features of interest, but beside this, new areas can of course be added, either adjacent to San Andreas or accessible via air travel.
The world as a whole would receive a significant graphical improvement. Where necessary, polygon counts would be increased, and texture quality would be increased. General engine improvements would result in even more realistic lighting. Suffice to say it would be the same place, but it would be noticeably more realistic, which I think most of us can agree would be saying something.
Let's not forget physical realism. There is no reason that the world can not become more interactive, with the ability to pick up, push, throw, dent, break, mow over, scatter about, and utterly obliterate the free and fixed objects of the world. After all, violence is more fun when it leaves a mark, right?
In short, I think the creation of San Andreas was a marvelous feat, and I think that, even as it stands now, there are a wealth of gameplay opportunities for such a painstakingly fleshed-out world. I don't mean just within GTA, but entirely different genres such as first-person shooters. I'm not kidding when I say that such a world could be leased out in the same way that land is leased in reality for people to create games within (not that that would happen).
GTAV's San Andreas is astonishing, and I think the work that went into it should be built upon and augmented in order to offer the greatest gameplay opportunities possible.