a virtual city is way more detailed than a virtual wilderness/wasteland. in gtaIV world more things/elements/aspects have to fit together in a functional way, while in RDR such constrains are much more permissive (smaller towns, fewer routines, etc). rockstar can't possibly rush a big city just to please eager fans. so i don't think more than one city would be... how should i put it... feasable right now. however, if it will be true that for a first time the land is not an island, but a continent (thus half ocean, half desert) i'd expect them to simply restrict the player from going on the other side of the mountains, but also have a blocked tunel (in construction), like that one from shoreside vale in gta3. this would give rockstar the "hinge" to unblock a new area of the map with a new dlc and a new city, like las venturas or san fierro.