The thing is, not every building needs to be an individualized work of art. For most of the space-filling scenery, it would suffice to model some generic, simplified structures chopped up into modular segments. These segments could then be copied, pasted, and re-arranged as needed to create non-repeating patterns. With one "set" of building types per district, plus a smattering of individually modeled "centerpiece" structures to keep things from looking too homogeneous, you could easily fill out an entire city. Just a thought.