Agreed. Quickly refreshing myself on Kezan/Undermine lore, the capital city is described as being subterranean. Going by the snippets we see of the surface in the goblin starting zone and Motherlode, there are large areas of construction and industry, but the rest of the island is still covered in jungle.
An entirely redone/revamped Kezan would likely consist of those elements - significant portion of goblin ruins leftover from the Cataclysm, some areas being rebuilt by returning goblins, large areas of wilderness, jungle (likely reclaiming some goblin ruins) and of course the volcano. Instances would take place below the surface in Undermine proper, with goblin construction as expansive as Blizz feels like doing.
I'm hoping for something that feels like Blackrock Depths, if not actually quite as twisty to navigate.