A couple of posts have pointed out the linearity of the city progression. I'm fairly sure that the design team have plans for this in place already, but I'd like to make a suggestion along this line as well:
Have some buildings use a special resource to build, similar to how a city has a particular number of enchantments that can be build on it. The number of "locations" available to a city depend to a large extent to the location of the city (similar to city enchantments), but a couple of additional locations can become available via city-population, research, quests, appropriate minor alliances, trading a city-enchantment for a location (and in reverse, building a location structure to grants a city enchantment), and so forth.
Such a design leads to city specialization as a given city might only be able to build 3 of the 20-or-so special location structures.
Of course the majority of mundane buildings do not require special location resources.