I mean, the big, expansive map, we've seen in games, particularly the ones you've mentioned. What we haven't seen yet are the big maps with expansive cities. That is, expansive cities with points of interest outside the city, rather than being the large islands we have with Saints Row 2 and GTA IV.
Of the sandboxes I've clocked significant time on, biggest city maps were SR2 and GTA IV. Neither of 'em had much outside the "islands" they were on, besides lots and lots of water. Though, I suppose it's feasible to turn part of that water into badlands...
Honestly, sand dunes, or dry cracked earth seems a whole lot less resource consuming than water, which would have to have the illusion of depth, and actually move, with splashing and waves and what not... it's surprising how much well done water adds to a game, and jarring at how poorly done water detracts from it... Whereas desert/ cracked earth, and even light forrest and hills seem much less resource intensive... and can seem every bit as endless as the ocean.
Again, ARMA 2 does procedurally-generated forest/scrub/hill/desert areas. This is not terrifically complicated or resource-heavy. Not to mention, it would only have to go on until you were lost and a pop-up appeared saying, "The wastelands of 2077 have claimed your life. Your corpse will lie here until one day it is over-run by the expanding urban terrain that, right now, lies too far away to preserve your life. Farewell, wanderer."
That wouldn't necessarily be a bad idea either...
Mountains with collapsed tunnels, The Southern California Border with AA embankments........
In fact, this would be my ideal opening for the game as a Nomad Character....
The game would start off-map. Dialogue, perhaps over a black screen or a video of some guys over a campfire talking about "the tunnel run". When game play begins you are riding along, manning a mounted weapon on a trailer, in a convoy driving down a long tunnel. As raiders attack you, you fight them off, then finally make it to the end of the tunnel, but a fuel truck behind you explodes, sealing the tunnel for the duration of the game.
The convoy of vehicles drops you off at the Nomad encampment outside the city. From there, you here rumors about life in the city. It costs money, or a pass to get in, so you ill have to do odd jobs in the wasteland till you get the scratch.
Once you do get inside, the city is immense. You make your way to the combat zone...
I still kinda think in my heart this is the best way to start the game period, as it provides a way for the character to be as new to the city/game as the player, so the tutorials and info dumps make a sort of sense in context... as opposed to having a character who has lived in the city all his life but somehow doesn't know how to operate the most basic of things like a dataterm, or where to go to buy food or guns or gear or whatever...