Well, the fourth approach is the best solution I can come up with.

I think it actually makes a lot of sense - people will relocate (i.e. change town of residence) freely, but not comute (work in one town but live in another)  Only downsides I can see is it may make the leveling of towns difficult to understand, and i'm not sure how the UI will work.  Perhaps the former population is now the town's prestige, and its the prestige that drives the leveling.

Since Frogboy is having some trouble with the new population system and is probably going to remove it, I'm going to make two population mods that, I think, should make population and cities more fun to play with.

1) A city's buildings are limited by the upper limit on available housing room in the city. A small city that can house 15 people would be able to build 3 improvements that require 5 population. Note the difference here between ACTUAL city population, and housing room. So, population can still "commute" between cities, but only if the recipient city has enough room to theoretically house the commuters. This will make it impossible to build a level 1 city and then fill it with a hundred studies or workshops. The player needs both the global population, as he does currently, and the local housing room, to build any building in a city. Armies are floaters, they do not draw housing room from any specific city but still draw global population. They can still be trained in a city where all the housing room is used up.

2) Each city, except the first formed city (capital city) will require 1 food to build and maintain. In addition, the infinite rule of repeatable buildings is removed. Currently, in-game, a city can hold a near-infinite amount of studies, for example you can have 10 studies in a level 1 town. With this mod, you can only build a specific amount of repeatable buildings per city level.

Level 1 city can hold 1 of each building
Level 2 city can hold 3 of each building
Level 3 city can hold 5 of each building
Level 4 city can hold 8 of each building
Level 5 city can hold 11 of each building

Coupled with the food requirement for each new city, this should give a very clear incentive to produce mostly medium or huge-sized specialized cities (no more "city-spam"). The bonus granted from city level-ups will serve to give efficiency to food vs population vs location costs.

But, most importantly, these mods will work with each other if the player has them both installed at the same time. I believe this will create a situation where the player will pursue highly specialized cities, as the 1) mod will limit what can be done in each city (you can't build all the arcane and tech research buildings in the same town), and the 2) mod will force the player to build effective towns. I believe the end-result will be a sort of dead zone in the middle, where the player will want just the right amount of cities, as too many would be costly food-wise, and too few would give insufficient room to specialize each city.

In addition, an unrelated 3rd mod will be included to further give incentive to building larger towns.

3) In this mod, higher level cities produce buildings and train troops faster natively, without the aid of spells or special buildings.

Level 1 city = 100% training and building time
Level 2 city = 90% training and building time
Level 3 city = 80% training and building time
Level 4 city = 70% training and building time
Level 5 city = 60% training and building time

Existing improvements to these parameters will be tweaked so the player does not reach critical levels where buildings and training finish unreasonably fast.

I believe there is a hard-cap range beyond which no resource will link to a city. I will experiment with it.

I did some quick tests with the 2) mod and it's really causing every city to look the same. The improvements don't cost enough to demand that the player specializes. Since you are required to build housing room in order to level the city, by the time you have to choose what to build you always have the population to build everything. I'm thinking much higher limits would be in order after level 3. It is probably better to tweak the tile limits for cities, coupled with the 1) mod (and a +1 food cost for new cities) that should make it very hard to build low-level towns effectively.

I also tried increasing the Zoc, but frankly, I don't like it. While there wasn't a hard-cap on the range to the nearest settlement, it caused the land to become segmented way too fast. The game isn't balanced for a ZoC war, and the Ai REALLY isn't up to the task.

Current 1.3 beta tile limits for non-capital cities:

level 1 = 4
level 2 = 12
level 3 = 24
level 4 = 40
level 5 = 60

I guess I would change this to

level 1 = 2 (1 food = 2 tile per food)
level 2 = 6 (2 food = 3 tile per food)
level 3 = 12 (4 food = 3 tile per food)
level 4 = 33 (11 food = 3 tile per food)
level 5 = 62 (32 food = 2 tile per food)

However... this is really just doing what the 1) mod is doing anyway, but in a less elegant way. In addition, the tiles for improvements really aren't balanced for their efficiency. Some will cost 4 and do little, and some will cost 1 and do much.

I guess the beauty of the 2) mod drafted in the previous post is that it sets a pretty high limit on how many studies you can effectively build per food. But the downside is that it leaves room for a lot of other buildings to be built instead, which leads away from specialization.

Re-thinking the 2) mod from a perspective of food cost vs study cost, assuming the player always chooses the city to improve on tech research

Level 1 city can hold 2 of each building (2 per food) (max research: 2) (research per food = 2)
Level 2 city can hold 4 of each building (2 per food) (max research: 4*1.2=4.8) (research per food = 2.8)
Level 3 city can hold 8 of each building (2 per food) (max research: 8*1.5=12) (research per food = 3) (libraries can double)
Level 4 city can hold 22 of each building (2 per food) (max research: 22*1.9=41.8) (research per food = 3.8)
Level 5 city can hold xx of each building

This way, a specialized city can outproduce a non-specialized city in food per research cost, especially if secondary buildings come into play that boost % further. But... already at a level 3 city we have a city that uses a massive amount of tiles. Cities will grow huge and cover the map again.

Maybe the solution is to have each city level natively increase the efficiency of the food vs research cost of each built study. I will have to experiment some, brb.

Alright, please comment on this solution:

Citylevels automatically increase the efficiency of pop vs output of the repeatable buildings (through level-ups). But, consider that a level 2 city would automatically employ MORE men in a study and produce a higher output. For example

Level 1 city = 1 study costs 5 citizens, produces 1 tech research
Level 2 city = 1 study automatically costs 6 citizens (20%), produces 1.2 tech research
Level 2 city = 1 study automatically costs 7 citizens (40%), produces 1.4 tech research
Level 3 city = 1 study automatically costs 8 citizens (60%), produces 1.6 tech research
Level 4 city xxx

What does this solve? The cities will be reduced in size on the world map. Before, you needed 30 tiles to produce 1 base research. Now, at level 3 cities, you'll need only 19. So, a massive amount of studies won't cover the entire map anymore. This way, we won't end up in a situation where cities grow too big. Indeed, it is imaginable to further increase the city-levelup modifiers. The repeatable building limit per city would be tweaked for this, so that there is a situation where high-level cities automatically (without choosing levelup bonus) gives you a better FOOD per TECH cost, but with this solution the result is not huge megacity spam.

It's also a boost to building times for leveled cities. A problem now is you almost always build another city sp that you can max out your used pop faster with an extra building queue. With this solution, leveling up a city actually gives you a free increase, which makes it very desirable, particularly to level 3.

The plan is to make this auto-upgrade happen for workshops, studies and arcane labs. The upgraded version of the study the library, will be removed. I don't even know why that building is in the game, it is grossly overpowered.