I think Humankind have a problem with strategic dominance, Industry focus is probably the best option in many cases. Yes, some times certain culture is your best option for an specific strategy, but usually if you need more of something, just build an aditional district to deal with it. A gameplay based in population makes the city expansion more accurate. Actually, you can have a city with more districts than citizens and stay verry productive, but how? Who is working in those inhabited districts? You only need production to build more districts. Lack of food? Build farm quarters, do you need money? Build market quarters, your sprawling city is losing stability? Just build garrisons or theaters.

This design also makes the industrial focus cultures the usual best option and cultures in the agrarian group, one of the worst. One of the bases of the game if “evolve in way you need”, look for the best option to adapt your strategy, but usually more production does everything you need. My proposition is simple: make citizens necessary for districts to operate. Every FIMS district gives citizen slots to the city not only to gain space, but also because the district needs the population to work. For example:  

You have a city with your base administrative center with 2 of every FIMS slot, 2 farmer quarters with 2 farmers slots more and 2 makers quarters with 2 workers slots. In your city you have 10 of population, you can fill the 8 base slots and choose where you put the last two citizens. If you put your two citizens in farmers’ slots, your makers quarters don't gives their industry bonus.

 

Also, to simplify, you don’t have to assign population manually to every district, just put in the slots you want, and your city will prioritize the more productive districts of that group. This system tries to solve some issues I have with the cities:   

• You no longer need to build more districts if your population doesn’t demand it. This allows you to build infrastructures or hold festivals until the population grows. 
• Agrarian cultures become more important. Usually, I choose agrarian culture because I like their history or in order to make a synergy with other cultures, but rarely because by themselves.
• The IA don’t need to build excessive big cities and are less probably they destroy potential natural reserve zones, so, in the post-industrial late game not every IA city your conquest is condemned to have periodical climate contingency.
• If your cities spread less, for the late game you will have more space to make space projects or nuclear tests (at least to me it’s hard to find a place and usually it’s an island.)
• With more unsettled natural land the late game wars can have more variety. Usually in the mid or late industrial era, many land battles are sieges because the cities have less space between themselves. Also, it’s hard to make a bombardment without collateral damage in a district.

Personally, I don’t know how this mechanic could affect the garrison quarter. Commons quarters just don’t apply their bonuses without at least one citizen, but in the garrison. Maybe it could represent the maintenance personal of a fortification, but if this happens maybe this district have to produce industry or something, like the Stronghold of Medieval English culture.