I really like the idea of smart algorithms that procedurally/pseudorandomly generate content as you walk, but this would be a pretty big hurdle as one would need to somehow merge the random parts of the map with the unique ones seamlessly. It would also make balancing the game kind of hard (provided that you would encounter enemies in the random parts of the world), as if you would just decide to jog to the first town, the time it would take you and the mobs you would kill in that time would make you overpowered by the time you reach the new town. Not to mention a plethora of other issues.
There are walkarounds to all of these problems, but that would entail a very different kind of game design from the way bethesda is operating now, or any other developers who make open world rpg games.
Well, merging unique and procedural shouldn't be much of an issue, it has been done in Daggerfall as well. You'd start with a basic map with two types of zones, random and unique, add more layers of basic height- and road-maps, general shrubbery and generate the map. Or, since Fallout takes place in a world mostly based on the real world, one could start out with a real map of cities and villages, topography, and roads, add layers for enemy density, shrubbery and so on, and let an algorithm generate the non-unique terrain to fill the world, constricted loosely by the starting topography.
With modern computational power you could probably even make a realistically aged map: Timed forest growth (ok, not really happening in Fallout), erosion, random events that alter the map until the map is finished.
Balancing is of course an issue, but I guess the monster density would be much lower than what you'd see in the usual Bethesda game. The world would be realistically empty, which would probably piss people off, but you're not actually supposed to walk everywhere in real time as you would need hours and days to reach your destination. Depending on luck and and skill you'd get a certain chance of random encounters on your way, just as in Fallout.
Well, I'm not a programmer or game designer so I might be a bit naive, but I don't see a fundamental problem with a concept like that. It's basically just blowing up the ordinary map of a game like New Vegas to realistic proportions and filling the voids with procedurally generated landscape.
Traveling should probably be made realistically hard, then, too. Like, requiring a certain amount of provisions and supply (so a food/water and sleep system would be necessary, or at least water like in Wasteland 2) to travel, restricting the player in the beginning to few locations, making remote locations only accessible by joining a caravan or later getting a car(t) for yourself.
I think the biggest problem is that the world is mostly empty, which will put people off, especially those into the "exploration" of the later Bethesda games. Exploration is much less fun when you don't run into a new interesting ruin every few meters and basically see nothing all the time (which is why Daggerfall had easy fast travel).
