Well again, Undermine (if implemented) would be the most well-known lore location in this expansion. Every other new location originates from TWW, and TWW alone. It is also a location they've wanted to implement since Vanilla. That could be an impetus for Blizzard to bring it into the game and make it a zone-sized city and break a few barriers.
I also don't necessarily believe that they would build an expansion around Undermine, and Undermine alone. There's enough material for a patch, but having enough material for an entire expansion would be a stretch.
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Because there's no reason for him to be there. Gazlowe is getting Undermine information from Goblins in Khaz Algar. As a trade prince, what happens in Undermine affects him, since it the center of Goblin civilization and the cartels. That in turn could influence Thrall, since Gazlowe is a Horde faction leader and the two have been friends for years, and Thrall is directly responsible for Gallywix remaining in power after the events in Cataclysm.
Mekkatorque really doesn't enter the equation.