Whenever I think about this subject, I feel that the tension is always going to be “how do we prevent the game from devolving into the ‘ecumenopolis simulator’ where players just pave every tile” with a solution that isn’t civ6’s 1 specialty district per city.
As usual, you put your finger on one of the main problems with any system in 4x games - how to keep the gamer from running out of control with virtually any system or mechanic provided.
That is humankind’s biggest failing to me - the system pushes you to make a mega city where you’ve coated an entire province with makers quarters and then you do whatever you want with you 1,000,000 production.
And was one of the things I brought up during play testing for
Humankind: that from Ancient times it allowed you to build Megalopolises that were physically impossible until after the Industrial Era, and totally out of place before that - and unbalanced the game into a sort of Strategic City Builder instead of a 'real' 4x historical.
Part of me thinks the solution is tying yields to pops and not letting static outputs be too substantial. (Like 6’s buildings do, sadly.) Civ has always required pops to work tiles, and one could have urban yields *mostly* tied up in specialists. So then I might want to place villages or some districts out in the area to access tiles, and only once I’m urbanizing do I want to build up the city center with more districts. (Presumably a player can’t afford a heavy specialist economy straightaway.) Tack on a population based limit on how much district you can build, and that should keep things roughly in check. You could even limit the housing space (and specialist slots!!) provided by the urban core until the industrial era, forcing players to build out the small farms/villages/towns so pops have places to live and jobs to work until then. And unemployed pops would be very unhappy.
Just a spitball of the mechanisms one might need to keep things under control - because players like me will always try to go nuts make some hyper industrial hellhole if given the chance.
Spitting out some more balls here, and building on your comments, because they parallel many of my own thoughts:
1. Districts from the start should be limited by transportation technology: no city can grow larger than the distance people can travel in part of a day, or the city becomes separate entities and unravels. As my sister the Doctorate in population geography would put it, people gravitate to a center where there are jobs, amenities, and protection, but the counteracting force is that only so many people can fit into a square kilometer of ground until you can build higher (wood truss framing, then metal framing, then skyscrapers, all Medieval and later technologies) so there comes a point where the concentration covers too much ground to sustain itself.
In game terms, I propose that initially all Districts have to be adjacent to the City Center. Period. That limits the Ancient, Classical and Medieval Cities to 7 Districts total, unless they have later tech that allows them to use water transportation along a river or coast to extend an 'extra' tile along that coast. In extreme circumstances, like when the city is at the junction of two rivers, that might add up to 4 or more Districts, but then only with Medieval canals to control the rivers. Once late Industrial Era Powered Transportation (steam railroads, underground subways, electric trolleys/trams, etc) is available it allows building out in all directions - but at an increasing cost per District for all the rail, tunnel, electric power distribution technology required to access the new Districts. With the Combustion engine and the personal powered transportation of the automobile in the Modern Era all restrictions are largely gone: you can build your Megalopolis and pack 10, 20, 30 million people into whatever number of Districts you can add - at an increasing cost in Concrete road construction, because unpaved roads will not support the traffic generated.
2. I would very much re-emphasize Specialists compared to Civ VI. I think it is the most neglected mechanism in the game for generating real Yields from the city population as opposed to artificial 'adjacency' and other bonuses. I would have every point of population generate both the usual pop point to work tiles around the city, and a Specialist point to work the buildings in the city. Don't build enough working structures, and your Specialists are out of work and unhappy (cue "Workers of the World Unite!" in multiple languages). Have empty Specialist slots in your buildings, and it will stimulate immigration (population increase) into the city.
I would like to see almost a mini-game of Specialist Use, because I think this is a better place to put all the various bonuses to the outputs of buildings than 'adjacencies' (although there is certainly a place for adjacency bonuses, I just don't think they should be more important than the Workforce). EVERY building should require a Specialist even to get its most basic output. For instance, Walls have a Specialist slot - who do you think shoots the arrows and catapult bolts that give Walls their defense firepower? And the effects of Specialists should increase as the game progresses. Higher Tier Buildings should generate more Specialist slots, and get more yields out of them. An early Workshop might have one craftsman working in it; an Industrial Era Factory might have 3 Specialist slots and generate 4 - 5 times the Production of a Workshop - with additional gains from Power plants and adjacency to Harbors, Airports, and Railroad Terminals to distribute the items produced.
The quality of Specialists - and, therefore, the output per Specialist - should also be Variable. The earliest Specialists might be largely illiterate and home-taught. A Specialist in a Research Lab of the Modern or Post-Modern Era, on the other hand, might require a University or Technical College, but will 'produce' far more Science (points) than the Scribe in a Temple School in the Ancient Era. There should be real and important relationships among Specialists, the Buldings that produce them, and the buildings that require them.
Differentiating the population like this, by the way, automatically provides the framework for 'Classes' of the population and all the potential internal friction that come with them. In the Industrial and later Eras, it also provides some basis for giving the warmongers amongst the gamers the real problems of mobilizing for war: an increasing percentage of your military manpower to man 'modern' weapons like Artillery, Tanks, and Aircraft MUST come from your Specialists, and removing them from, say, your Universities and Factories will appreciably reduce your Production and Science output. Fail to find a balance, and your military specialists are left without weapons or your weapons are left without effective crews. (And yes, this is based on an academic work: Tooze's
Wages of Destruction, a massive study of the German economy in World War Two, which massively failed to strike a balance between what it wanted in its military and what its industry required in manpower, and so failed in both areas)
3. Population 'limits' on cities are historically a combination of How Many Can You Feed? (always primary) and How Many Can You Keep Happy? Part of the latter is 'inherent' in any city: increased access to amenities and goods that just aren't available in the average farming village, like entertainment, culture, protection of walls and troops, access to Power and the market for finished goods that is the Palace or City Administration, etc. Primary population increase in all cities should come from Immigration, so Amount of Attractiveness, as in Cultural, Entertainment, Religious Buildings, availability of Jobs (slots in Buildings), Food, etc. should be a major factor in population numbers. The Palace would automatically be a big population draw, because the Palace complex was always the top market for luxury goods and, of course, access to Power and Privilege.
