My LS and I have talked about this a lot also....
I always figured most of the "fighting" would take place in new zones
Maybe thats why Ishgard wasn't opened...
Maybe it doesn't play well into the "prequel" of it all
The biggest problem is the whole main scenario quest from the city-state quests all the way to R46 talk about the impending danger... If they have all the climactic stuff happen in as of yet unopened areas then they minimize the amount of stuff they actually have to change