Then you have to consider that disconnected zones never seem to work for WoW, so you combine the zones.
And hey presto you have a world revamp. You go to a revamped version of Searing Gorge where you fight Dark Iron Dwarf rebels instead of just regular Dark Iron Dwarves and generally have a good time in zones you enjoy.
Uldum and Hyjal, and to a lesser extent Twilight Highlands already existed on the map back in Vanilla, so you didn't have to stretch as far to say why you never visited these areas.
Now there are not really any areas like that. You could have islands, but that is just again not really solving anything compared to reusing old zones, giving them the Darkshore or Arathi treatment, and then using them as practically new zones.