There's no way it would be this gen systems. Probably not even next gen really to make it that real and that big you are looking at pretty much all those individual game making time in one. Which is like 10years. Gta San andreas was pretty damn big and was so fun flying planes around. Only thing with it is there was a lot of farm land. You could get lost for hours with all the empty mountains or fields between cities. However making that game and changing each city section to something more different from one another. Like one is more city, another is more hood, another is more vice city, etc mixed with today's graphics I think is more possible. Basically all mini cities of the old games in one. Would be pretty awesome.
Personally I really wanna see a true drug lord game. I'm not talking scarface selling dope than buying cars and clothes. I want true thousands of killograms of cocaine to various countries. Having to Dodge police,set up car bombs, live in hideouts, create smart ways to traffic etc would be awesome.