Yanno...
Maybe it'd work best as a gigantic open sandbox game. I was playing GTA V a while ago and thinking that with some tweaks and mods, the game would work amazingly well. Even Los Santos, the way it is, is almost perfect as a location. You've got a smallish town near the ocean, big city, mountains and lakes, woods with a lumber yard, a military base, and so on. Main issue would be that the game is primarily designed as a shooter. Melee combat and stealth are just kind of in there so you can mess around with them if you want.
Man... If Rockstar did this, that'd just be the most perfect thing ever.