Certainly, they were very important historically. But, my great takeaway is that, given the current systems,
I don't think the way CK3 was developed makes it the correct game for them, because they would require too much reworking of too many tied-in systems to be fully functional and rewarding.
I've tried out some of the older mods that used to have Pops attached to them, and, quite frankly, they didn't add any in-game complexity, even when they matched the level of depth to what you described. They were just a modifier that gave more taxes and levies, which is what Development already kinda represents. I got events about fires and earthquakes, but, after, 5 years, I stopped caring about the number that went up, down, up, down, etc, because having 1,000 burghers or 10,000 farmers didn't contribute as much as Manor Houses. There's no character attached to them and there wasn't any real tragedy or cost associated with them--I felt more impact from the mod that made it so that sieges could
destroy buildings and actually damage my economy.
Inviting farmers to work your fields; you mean spending money to get an income increase? That's called upgrading a building. The economic systems of CK3 already have better terrain baronies
produce better economic buildings (manor houses > farms & fields > most buildings > forestry/desert agriculture), so the pops would already flow into this dynamic. What would it take to attract pops away from high development provinces that would be fun to interact with? The game just represents the Ostsiedlung with a Development and Cultural Conversion bonus, and, without any kind of event spam of "GERMAN SETTLERS WANT TO MOVE IN, JAH ODER NEIN?" the level of interactivity isn't that exciting.
So, addressing the above, unless a complete economic rehaul was made involving pops, the building system, and development, the best we can get is a loss of income due to low Control after an enemy attack. Paradox said they'll be adding merchants [republics?] in Chapter V, but I don't think they'll go so far as to implement EU5's Goods mapping.
If anything, as you mentioned about Freezing buildings, or at least reducing their efficiency pending an amount of pops needed to work them, could be a stopgap, sure, though it's a kind of binary (not met = worry, met = ignore) kind of issue.
Meanwhile, what if pops were connected to levies/MAA? Well, the creators have already said the reason why levies replenish so fast is because the AI can't handle their armies and, after losing them, would just be dogpiled into oblivion. The same could be said for pops, as well, with AI unable to manage their military and pop growth, and, thus, collapse into 100-person counties as plagues sweep through their lands, their cities deserted without peasants to feed them.
Meanwhile, a large kingdom would always just be better because more land = more pops; playing tall would require some very careful gardening to prevent plagues and other sources wiping out your workforce and be unable to resist larger neighbors--would being unable to resupply your MAA due to a vast number of factors out of your control be a fun feature? Or would it just be a noob trap that makes new players quit?
Would having local food supply contribute to a siege's length be a great feature? Yeah! But, then again, high economic buildings provide greater development which makes for stronger counties to put money into stronger defensive buildings. Supply is a flat number and barely gets touched, even if a 100,000 strong army sites there, laying siege for 36 months.
Looking at the military side, this too, would mean a full military, building, supply, and AI overhaul would be needed, which, quite frankly, has been a day 1 complaint here on the forums.
At this point, we're already tearing out and re-wiring two base systems which the AI presently struggles to use correctly, and making them more complex so that some background numbers can give us more income in our high development/good terrain provinces. Now add in whatever systems All Under Heaven are going to change to the east, and there's another array of complexity that'll probably need its own beast to deal with.
This is even before we start conceptualizing minority mechanics,
treatment of relgious sites, and what associated kind of interactions could be used for pops, as we presently just have Control and Public Opinion. That means more menus, more granularity in religions and cultures, and more things that detract from finding marriages, going hunting, or launching wars. Hisyoric taxes provide a lot of data to study, but having to scroll through submenus to alter a city's market contract rights can be burdonsome. Representing the history of things like Frederick II's Lucera Archers or Köln's protection of its Jews during the Rhineland massacres are interesting displays of cross-religious interactions, but what would make it worth the player's time when archers are a poor MAA and Cologne's Farms and Fields produce more income than its minorities?
Meanwhile there's plenty of action like the mobs of Rome affecting papal elections, the burgher's revolt of the Jacquerie that nearly upstaged Paris, and the countless peasant movements of China during times of turmoil. Then there's how Traveling populations would work, particularly with nomadic realms and unsettled baronies. All of the calculations wouldn't help players struggling with all of the current Administrative schemes, let alone what'll come with China, and then there's the event spam of "10 Norse-Mashriqis were killed in a feud with a powerful Ethiopian family. Will you spend 500g or lose 10 Public Opinion from the Norse-Mashriqi?" These would then all have their mechanical implimentation, because a 10 year modifier for "bad weather" is just silly.
If they had been idealized from the start, like how EU5 shifted away from EU4's Mana-based Development (which, in itself, had been added via an expansion that has now been integrated into the base game), I think it could be possible, but, unless serious dev time is taken to get under the layers of CK3's exterior I think it's much more likely to be painted on a fresh canvas in CK4.
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