As far as I understand how Blizzard designs their terrain, I estimate the actual formation of the zones is the least of their effort. It's the populating of these zones with points of interests that requires real effort.
Procedural design has been present since Vanilla. Desolace is the clearest example. It shows eroded hills and also the way the textures run around the hilltops to simulate water taking the sediments down:
So a designer starts with random hills, then sculpts in the terrain according to what the lead for that zone has in mind, and then it has an erosion pass creating these beautiful textured streams, and then probably still needs a terrain designer coming in to correct any wonky textures that came from the process.
And this is Vanilla content, it's old. Terrain generation has advanced since then. Blizzard uses new techniques, but the point remains, it's a mix of procedures and hand-sculpting that pushes down the development costs, while at the same time every actual asset and story has to be built from scratch.
Imagine it like raisin bread. The raisins are the content, the dough is the terrain. You can chose to add more baking powder or yeast (I'm no baker) to let the dough rise further. The amount of raisins will stay the same, but they'll be scattered further apart from each other due to there being more terrain that fills the space in between.
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Sounds amazing to me. Especially when these events impact each other in a sequence. It's not just rolling the dice for each city, but each city having an entire string of outcome dependent dice forming its history. Maybe an earthquake caused a war, and that war prevented a faction from conquering and enslaving another faction that now went on to invent some technology that created a disaster in another place etc. Uncovering that kind of history is thrilling, and no WoWhead guide will spoil it for you.