It is definitely possible but it would mean adding some sort of tunnel to transition between the areas using different art assets. Though tunneling is mentioned in the his discussion a lot it only refers to limiting what the player can see and for how long so it can dump memory and load new memory for the other area. It's not always a tunnel or straight path, it could be a fairly wide space as long as there aren't many textures and models being used in this zone (just stating this part for others, I know you know this).
But since they have increased mount speed and flying is much faster, it makes it harder to transition between maps seamlessly. Basically meaning they wouldn't have been able to increase mount speed if there was seamless transitioning because then you could get through the loading zone faster. And I don't know how flying would work with that since you can't limit what they see unless they literally make a tunnel that leads to the other area
I think at some point they say they may want to make flying possible in 1.0 areas but they aren't really designed for that. If they did do that they would probably make the maps a bit larger like how the 3.0+ maps have been but I can't see them moving away from the isolated maps we have now only because they decided they would be that way when they were creating the game based on player feedback on 1.0
Most of my game dev knowledge is on the artist side rather then the programming side so if there are more technical tricks then that, I might not know of it. I know mostly about the limit on art assets based on memory limits on the system it's running on, and how the assets are used through out the map to hide how much is being reused depending on how big the map is.
I can see them removing zones in cities possibly but not between open world zones unless they did a lot more work that wouldn't really benefit the game much. So most likely they wouldn't do this unless it was a more drastic change like revamping the areas to be designed around flying and swimming because then they would already be in there changing the map