Next Expansion: Underground Zones

The two major enemies of Azeroth are the Burning Legion and the Old Gods.  It's reasonable to assume that the next expansion will have either, or both of these two, as the main villains.  As such, to have more zones for high level play, Blizzard is going to have to find a new place.  I think several underground zones would be idle for the Old God part of the next expansion.  This would be a perfect place to introduce races not already in lore, classes not seen before, and have more lower level content to go with the higher level content.