I think if I was to do a Goblin-focused expansion with a Tinker class, to get around the problem of Undermine being a subterranean borefest, I would also only relegate it to a sub-zone.
I would also have the main feature of the expansion basically be building up a zone live as the expansion rolls on, maybe a shared capital city that gets bigger and bigger with each successive patch and starts to encompass more and more content around it. Really condense the playerbase and make sure you can control their actions at a higher level so you can force community shenanigans that you couldn't before. Let it start out as a simple hub, then a more expansive one, then expands even further to include quest content and even dungeons and raids.
Basically take the Gadgetzan concept from Hearthstone and run with it. A Goblin-built town for everyone, encouraging emergent player behavior. Could be cool.