The city generator can just be setup to spawn the actors in your scene (roads, buildings, props) and then you can change any of the actors in your scene after the fact. That part is “relatively” easy to do. What seems a bit harder is adding randomization to the city generation - do you want to generate a random road network that makes sense? Do you want to randomize each city block? Etc, etc.
The example from Sinking City is actually a happy medium IMO. It looks like the artist defines the road network (no randomization there) and then the tool populates the surrounding areas with random assets based on a bunch of parameters (like building size, orientation, city block type, etc). A tool with this functionality is fairly feasible I think
