I think considering how much effort was spent on the seamless loading in 10.1 from going underground that said tech will play an important role moving forward in this upcoming expansion. Either seamlessly loading different islands, going through levels (above and below ground) and such.
We know it's not pirate themed because of what the Blizzard prez tweeted saying it wouldn't be Pirates but perhaps traveling between island landmasses or even above and below ground zones could see more attention in the expansion. Just an assumption.