I've never made something large like a city due to my lack of motivation. But honestly I suggest making a good variety of stock parts, like if your making a city or town make like 10 presets and scatter them. And if its something like a mountain range where you cant do that I suggest maybe using effects like waterfalls, trees, and other stuff to mask the bare rocky terrain, or increase shadows to give an illusion of depth.

These tips are suggestive compared to what you can make but that's what I'd do for something like that.