Adding flying to old areas will present the same issue that it would in WoW back when they did it. There are a lot of areas we can't see over, like the rock walls in thanalan for example, there isn't really anything behind those, it's just untextured empty space and even if they put an invisible wall there it would look really ugly. La Noscea wouldn't be as bad because of the water areas but they'd still have to texture the areas we can and cannot go to but I doubt the devs want us to see an empty void or a sudden drop behind the rock walls since there's no textures behind it.

But if they do this revamp, I actually hope they make the world a bit more seamless. Like being able to fly from say aleport straight to the isles of umbra or fly to the ships for the sahagin dailies rather than taking the elbst. I thought about them making a idyllshire a part of it's zone but after thinking on it more, I think idyllshire should stay as is.