As the city grows, there are more citizen. In other transportation games like transport tycoon your incentive to grow a city is that it will consume more goods for whose transportation you get more money. There's no concept of money in Factorio yet.

What kind of benefits do you want to get from an increasing population?

Currency to invest in:
Research for better (Larger?) trains or fuels.
The supply chains themselves.
Other parts of the logistics (Such as loading/unloading stations)
Perhaps even interior train design. (Referencing to Comfy Factorio's mountain fortress game mode here, if you never heard about it: https://github.com/ComfyFactory/ComfyFactorio/ )

Specializing population in the real world required a higher population count. The need for hunters for food is a lot higher than the need for developed roads. Kurzgesagt says it a lot better than me... https://www.youtube.com/watch?v=QsBT5EQt348

So tapping into that knowledge, use that as a game mechanic in your mod. Access to higher researches only be available when you reach a certain population count; You could use trigger-based researches for this (Check Nullius mod for reference). Tapping into this system also allows you to tap into more specialized demands. You create a positive feedback loop like this where the player wants the population to expand beyond just "I wanna see number grow".

The more I think about it, the more I'm onboard with the citizens generating some currency (i.e. taxation), that can be exchanged for other things.

There are some technical details to work out, for example if the currency is just a global number that ticks up, or if it's an ingame item that you can for example get from a bank and then use as a recipe ingredient.

That currency could be exchanged for
- unlocking specialized research (think new research items that need "citizen science"),
- purchasing specialized items (e.g. train stations or passenger trains),
- funding primary industries (if we go with an approach where the player can't create but only discover them; you can mine iron, but you can't place it),
- and late game research.

Specialized research could unlock items that are only part of the mod, such as higher quality luxury items that you then need to produce for the citizens.

Purchasing specialized items is something that I'm not fond of yet, because it deviates from Factorio's general approach of crafting everything from the few basic resources. It feels odd that you would have to use "currency" as an ingredient for a train station, when at the end you're not paying someone else to do the work, but it's you or your drones (who work for free). I'm currently just thinking about items you could place in the world or equip to your armor, maybe there's something else?

Funding primary industries is something you could also do in OpenTTD. By default there are some primary industries that you can get e.g. apples or wheat from. If you use up all their output, you need to explore farther. In OpenTTD you eventually run into the end of the map. With this mod I want you to be able to explore indefinitely to find new primary industries. By funding new primary industries you might be able to pay currency so that new industries pop up somewhere randomly in already discovered territory, or somewhere that you select. That feels like it needs a very steep price tag so that you don't just put them everywhere, or it would would loose the transport aspect in late game when you have too much currency.

With late game research I'm a bit more fond of the idea, esp. with infinite technologies like mining productivity and the other ones announced for the space age update. I imagine a building like a university that you can provide currency and it then generates science for you. Then you can funnel that into other areas of the game, and have the citizens help you improve your late game further.

On the topic of unlocking certain research by population count: I'm on board for that with research introduced by this mod, but not for research from the vanilla game or other mods.

Since this mod will probably make the players lean into train transportation more than in the vanilla game, I'd like to give them access to those mechanics faster. The citizens might help with unlocking train tech and building some rails/signals/trains themselves so you can get started there slowly without having all the rail production set up yet. Is that something the mod should help with, or are trains accessible enough? Or should the city grow slow enough so that you can research trains quickly enough?

How about Introduce items "go-to-work worker" and "leave-work worker"?
It is little strange that butchery or university can produce meat/science, regardless of population, because they should need many manpowor.
So change recipes to require "go-to-work worker" and produce "leave-work worker", then player transport them to their houses, with belts, and houses genarate "go-to-work worker" from "leave-work worker" consuming food, water, etc. so player need to enlarge population, like need to make more assembling machine in vanilla.
This thing can also be applied to vanilla recipes, for example, introduce steel plant that produce iron plate super faster and more productivily, but require "go-to-work worker" in their recipe.
And ofcourse the project is very cool, so keep doing nice work! and just sorry for my bad english.

I like the idea, but moving workers back and forth with belts feels a bit off, because it's likely to lead to belts with massive queues, and not enough workers to saturate the paths to all locations.

a natural way would be to use bots, with (hidden) requester chests and active provider chests, to get people to their destinations, using vanilla mechanics. the looks of 'flying taxis' would clash a bit with the 1950s style, but... it'd work, and most importantly, it would steer clear of money and taxes, which don't feel like factorio at all.

you can have workers with different skill types / skill levels too.

I think money is not a bad element, but yeah, it may make adjusting game balances so difficult, because having too many uses for money might make this mod just a difficult-to-understand version of the black market mod.

i'm currently modding my factorio and game accross your mod and it just looks awesome, will be trying it out, hopefully with a lot of my other mods.....won't be taking your suggestion to play without biters currently as i am super interested in a survival horror defense required gameplay with just dumb extra mechanics like oxygen and survivor landing pods and a whole bunch of nonsense, but i want to see how your mod fits in with that idea. anyway, that's all to say, i haven't read everything or played your mod so if i say something redundant or ignorant that would be why. firstly; the city being able to start being self sufficient to a degree, by means of the manpower working to sustain itself through your management, and then utilizing any extra manpower as any governing body would--for resource, expansion, and conscription, and ultimately leading to a grand assistance towards space exploration (with the incoming 2.0 update, this will be super helpful i would assume for otherworldly operations). another opportunity if you know how to implement would be some kind of loyalty or happiness or health, ultimately leading to the decay, death, or rebellion of the city (cities) and starting an enitre chain reaction in the world causing factions to pick sides, war to break out, and lo and behold now we have hearts of iron and essentially sims/hearts of iron/tropico in factorio. wow. anyway, i know this probably is way too much, or some already implemented, or i will get the "just go play those games" response....but why do we make mods if not to literlly modify our experience to incorporate things from other games and hopefully make a more personalized and awesome playthrough? i wish i knew how to mod or code a game.

I really like the discussion here! Let me recap some of the early ideas and how it evolved so far.

The more I think about it, the more I'm onboard with the citizens generating some currency (i.e. taxation), that can be exchanged for other things.
There are some technical details to work out, for example if the currency is just a global number that ticks up, or if it's an ingame item that you can for example get from a bank and then use as a recipe ingredient.

Currency is now an integral part of the mod. Citizens pay for food and materials, and you can use that currency to fund research or cities. Funding new primary industries is not in the mod yet, but a possibility. Funding new cities may be reworked soon (see https://mods.factorio.com/mod/tycoon/discussion/657786b3df202d0b9899fcbd).

Specialized research could unlock items that are only part of the mod, such as higher quality luxury items that you then need to produce for the citizens.

This is now also part of the mod. You unlock higher tier food stuffs, so that your city can grow from one tier to another.

With late game research I'm a bit more fond of the idea, esp. with infinite technologies like mining productivity and the other ones announced for the space age update. I imagine a building like a university that you can provide currency and it then generates science for you. Then you can funnel that into other areas of the game, and have the citizens help you improve your late game further.

We now have a university that can produce science packs. You can funnel them back into other research, or further enhance the productivity of Tycoon's primary industries (wheat farm, apple farm, and fishery).

Since this mod will probably make the players lean into train transportation more than in the vanilla game, I'd like to give them access to those mechanics faster. The citizens might help with unlocking train tech and building some rails/signals/trains themselves so you can get started there slowly without having all the rail production set up yet. Is that something the mod should help with, or are trains accessible enough? Or should the city grow slow enough so that you can research trains quickly enough?

The mod has evolved towards being a bit slower, and only needing rail transportation once you grew the city a bit. That means rail technology is usually researched before you start needing it for transporting passengers, or advanced resources for higher tier foods.

This thing can also be applied to vanilla recipes, for example, introduce steel plant that produce iron plate super faster and more productivily, but require "go-to-work worker" in their recipe.

I like the idea, but moving workers back and forth with belts feels a bit off, because it's likely to lead to belts with massive queues, and not enough workers to saturate the paths to all locations.

Moving workers back and forth for worker-powered factories sounds interesting to me! I don't recall the exact name, but there's a railway game where you have to transport workers to factories and back.

I'm unsure about how it feels to play with workers on transport belts, but it's something we can explore. Then the player can decide if the increased complexity is worth the increased output.

Do you think the new factories (like steel mills) should be placed on the map by the mod (as we do with primary industries right now), or allow the player to pick a location? I probably want to enforce some distance from cities to keep the idea of transport tycoon alive.

i am super interested in a survival horror defense required gameplay [...]
extra manpower as any governing body would--for resource, expansion, and conscription, and ultimately leading to a grand assistance towards space exploration (with the incoming 2.0 update, this will be super helpful i would assume for otherworldly operations).[...]
some kind of loyalty or happiness or health, ultimately leading to the decay, death, or rebellion of the city (cities) and starting an enitre chain reaction in the world causing factions to pick sides, war to break out, [...]

My vision for this mod is more about transportation and growing cities. We may introduce workers as a reward for supplying the city, which let you be more efficient in your production. City rebellions and other effects don't really fit into that picture, but they are a cool idea! Maybe the developers of Nauvis Post Collapse may implement something similar. You should check out their mod :) As for happiness, we are heading into that direction with various updates of the growth mechanism. Decay and death is something I'm still cautious about, because I don't want to introduce a negative death spiral.

I'm also excited for Factorio 2.0 and how this mod can fit into the space age theme!

Considering the many extra train functions in 2.0, I think it'd be great. Also, merry Christmas!

i am super interested in a survival horror defense required gameplay [...]
extra manpower as any governing body would--for resource, expansion, and conscription, and ultimately leading to a grand assistance towards space exploration (with the incoming 2.0 update, this will be super helpful i would assume for otherworldly operations).[...]
some kind of loyalty or happiness or health, ultimately leading to the decay, death, or rebellion of the city (cities) and starting an enitre chain reaction in the world causing factions to pick sides, war to break out, [...]

My vision for this mod is more about transportation and growing cities. We may introduce workers as a reward for supplying the city, which let you be more efficient in your production. City rebellions and other effects don't really fit into that picture, but they are a cool idea! Maybe the developers of Nauvis Post Collapse may implement something similar. You should check out their mod :) As for happiness, we are heading into that direction with various updates of the growth mechanism. Decay and death is something I'm still cautious about, because I don't want to introduce a negative death spiral.

I'm also excited for Factorio 2.0 and how this mod can fit into the space age theme!

This mod kinda has a potential to add "space colonization" to the game. Which could actually be a decent lore on why are we even here.

Economy is interesting, as the world of Factorio is 100% post-scarcity as the Engineer casually pulls up a clanking replicator (a version of the gray goo, literally a normal factory producing it's own parts for expansion.)/santa claus machine (a factory that can make virtually anything).

Why isn't it done by any of those other people? This detail can be ignored for gameplay, but if you can figure out how this world works with your new rules, then you made it a lot more immersive/alive.

Is there any intention of making the mod more viable with biters turned on?

It could be very interesting if you start with one city, and supply it so that it can send out expansion parties to take over resources from the biters. Some of the towns around the outside limits of your expansions would have different demands than those that are closer to center.

Is there any intention of making the mod more viable with biters turned on?

Yes, I'm open to ideas and suggestions :) I like yours! Others suggested cities building defenses itself as well.

I think that spoiling go-to-work citizens is a great combination (https://factorio.com/blog/post/fff-414). Cities produce go-to-work citizens, the more unspoiled go-to-work citizen arrives at the farm/factory/uni the more it produces, and player has to transport citizens ASAP.

You could even extend on this idea to have 3 tiers of citizens, where basic buildings (eg. farm) accepts all three but advanced (eg. university) accepts only to tier of citizens, but higher tier citizens spoil way faster. Furthermore smaller cities could produce only low-tier citizens at first, and after hitting a size threshold start producing some percentage of higher tier citizens (or always produce them across some exp curve where for small cities they are so rare it's not worth the bother).

On a mechanical level, I like the idea of using spoilage to measure this. This is exactly what the vanilla game will use it for.

On a level of how it looks like, spoiling people is ew. xD

I'll definitely take a look at spoilage when Space Age comes out, but sending people will probably not happen as I'm wrapping up features for 1.0.

Thank you all for the amazing input and all your ideas :)