I believe we'll be visiting locations relevant to dwarves (mountains), lizards (subterranean perhaps?), elves (forests maybe), humans, and maybe something specific to undead. Plus the spirit realm place. Should be a variety of locales and environments, but doubt an underwater city will show. They did a fairly good job of variety of environments in D:OS1, and the island seemed fairly interesting to look at in the release footage, so I have good hopes. They've talked a lot about designing stuff specific to the races, and Swen's inspiration of the dunes from his homeland for the island we saw, so it's obviously something they care about quite a bit. Not sure who the art director is.
I suggested underwater city under the dome and theese environments because the lore permits unlimited possibilites of the worlds and realms.
Maybe some of the dwarves left the mountains and settled in abandoned underwater city which poses the question who did live here? There could be some witty quest lines attached to this.
I wish spirit of theese locations will be in the game. Lorewise, they could be explored via realm portals.