Well... the two will kinda go hand in hand I guess, because the game is set up so it caters to every level increase. Why bother going to new zones when you've got everything close to the cities and in the same old zones, you know?
But if they restrict mob levels in the old zones so that high level players are forced to use the new zones, then that's just going to be vast, vast stretches of mobs and land that's going to be filled with cut/paste mobs and content.
Would just be easier if they redesigned the current zones before adding new ones. Maybe keep the climate similar, but break it down something like this:
South Thanalan: Rocky desert
North Thanalan: Sandy/Saharan desert
West Thanalan: Badlands or Savanna
East Thanalan: Savanna/Tree Savanna