So I think the overall quality of the maps could be greatly improved by pimping a bit what's actually outside the cities. Putting some trees and forests, using gradients on "non-built" ground to make it feel like it's not 100% flat, and maybe dividing the farmed areas into actual plot of lands (this last one might come hard to make look good, but it's worth thinking about, and maybe prototyping).
Here's a couple of Ideas
Is it really a detrimental botteneck tho? As an end user I would be more interested in a good end result rather than performance (if i where to choose). It's not like I need to generate 100's of maps in quick succession.
That is a pretty sick move to make it 3d :P
You have a good point about the unresponsiveness, but stating that in clear text to the user would be a quick fix for that tho. I mean, nobody likes to wait, but if you feel you know why you are waiting, most people are a lot more OK with it. Might be moot if the 3d method is that much better at drawing.
Hey, totally unrelated to this, but do you plan on adding rivers? Either through the cities or around the outskirts? With most cities in the real world springing up around them, I would think it a pretty good addition to this excellent generator.