One thing I would love to see in a city generator like this is a simulation of time and how gradual evolution shapes a city.

There's a picture book called "Barmi: A Mediterranean City Through The Ages" by Xavier Hernandez and illustrated by Jordi Ballonga. It illustrates a fictional city from Neolithic settlement to Roman outpost to medieval town to modern city. Some images can be seen at http://jordiballongaen.blogspot.com/p/baronia.html

In this book, you see how a Roman amphitheater decays during the Dark Ages, and the structure is partially used and cannibalized to create a circular set of apartment buildings, which then influences the later layout and design of the city in modern eras.

Procedural generation unfortunately tends to try to generate everything at once, as if a medieval city was created ex nihilo. It would be interesting to see what it would look like if such designs incorporated evolution over time.