1) Make citizens more individual, each with their own stats and combinations of them would decide the stats of the city and civilization. Make citizens an important cornerstone of entire game:
- Happiness
- Occupations - initially only 2 are available, farmer and worker, based on the type of tile they are working on (farm/pasture and mine/quarry/logging yard). Later farms and logging yards could become a plantation or camp, and citizen become a merchant. Additional occupations become available like cleric, scholar and artist. Each occupation have levels, citizen gets experience for working in their occupation, and after accumulating enough they can get promotions. level is automatically increased the longer the citizen works in their occupation. For example, farmer gets +1 experience for every turn they work on farm, so after creating a farm the citizen becomes "Farmer 1", after working on it for 5 turns - "Farmer 2", another 10 turns - "Farmer 3", etc. With increase in level they get promotions with new abilites and can provide more yields to the city.
- Loyalty to the city - Easier to change. Displayed as a diagram with loyalty to current city and all nearby cities of your civilization. If nearby cities are better, it would increase the citizen loyalty to them, and if loyalty to current city decreases too much, then there is a chance for the citizen to leave the city and join another one. The loyalty is increased by level of occupation (the higher the level the more loyalty), decreases by amount of trade routes to the other cities, etc.
- Loyalty to the nation - Harder to change. Displayed as a diagram with loyalty to current nation and all nearby nations and city-states. If other nation is better and is nearby, then citizen could lose loyalty to their current civ and migrate.
- Belief - also displayed as a diagram it shows the current belief and pressure from other nearby believers.
- Education - after creation of schools/universities every citizen can get an education and it could be increased with new buildings later on.
2) No separate workers/builders. City tiles are worked upon and improved by citizens. Appoint the citizen to flat tile and order it to build farm, it would do it in a turn (or few) and work on it, providing food to the city. Remove the citizen from this tile and the farm would decline and then disappear after few turns. Or you can order them to create plantation on the same tile, or pasture. Appoint the citizen to a tile with hills and order them to create mine or quarry. Appoint them to forest and order them to create lumber.
3) Citizens could be recruited from the city to become units and/or perform some other orders. It is easier to increase the number of citizens, but recruitment of the citizen removes them from the city so they no longer work on a tile and decrease the city's yields.
3.1) Units:
- settlers - based on their original occupation the settler give the new city a slight bonus towards specific yield. For example, if citizen was a farmer, then the city it established would get bonus to food production.
- military units - also gets slight differences based on their initial occupation, worker is a bit stronger, merchant is a bit cheaper, scholar gets faster experience, etc.
3.2) Many other tasks are IMHO would work better if done not by a separate units moving on a map, but from a menu like espionage worked in Civ 5 or like a governors worked in Civ6:
3.2.1) Trader - IMHO in Civ 6 neither traders as units nor trading resources with leaders made sense. Traders travelled too long, in-game timeline they could spend centuries to go to another city and return which is ridiculous, but immediately exchanging any amount of resources with leaders no matter how far their civilization is located is also unrealistic. So, instead of being a separate unit, trader becomes available in the menu. You can send trader to another city and appoint a limited amount of strategic/luxury resources to bring away for exchange. Based on their occupation the trader have differences, former farmer can bring surplus food from the city for exchange, worker would be able to bring more resources, merchant would get better prices, cleric would bring more religious pressure to another city and prevent pressure from another city's religion, scholar would be able to exchange not only resources, but also information (maps of explored territories, contacts of other civilizations, technologies), artists would bring more loyalty pressure and could also exchange information (maps of explored territories, contacts of other civilizations, civics).
There is no limit to the number of trade routes, as long as you have citizens and willing to turn them into traders. You can send more than one trader to the same city. Trade routes are established between cities, and can trade only resources available in these cities, but you can establish inner trade routes to accumulate surplus of resources in one of your cities and sent international routes from it.
Trade route's destination is limited by distance, but it takes few turns to stablish and few to finish (for example, 2 to establish, and 5 to finish).
Trader gets more loyalty pressure from the city of destination (and civ, if it is an international route) than normal citizens, and also gets religious pressure, if the city of destination have a majority religion.
Trader have a chance of being robbed and/or killed in the city of destination, so you would lose resources.
Trader level's up after finishing the trade route, and have their own promotions, for example, decrease their chances to be robbed, increase the amount of resources they can bring, etc.
3.2.2) Missionary - It could be appointed to another city there it would add +1 pressure towards religion this citizen is following. Missionary gets experience up for each citizen converted to its religion in the city it was appointed to and more experience once his religion becomes majority in the city. With enough experience Missionary get promotions.
3.2.3) Inquisitor - Can be only recruited if civilization have a majority religion. When appointed to the city, it kills missionaries of other religions, and it also provides +1 pressure for its religion.
3.2.4) Envoys/Diplomats and Spies - After finishing at least one international route, or converting at least one international city, trader/missionary could be turned into envoy/diplomat that would stay in the other civ's city and provide constant effect, or they could be turned into spy. Spies could recruit local unhappy/unloyal citizens to become spies as well and they would decrease their yields to the city.
3.2.5) City Leader and Governor - citizen could be appointed as the leader of the city and, depending on their original stats and promotions, can provide different bonuses to the city in the same way how Governors works in Civ 6. But Leaders could be further promoted and become Governors of several nearby cities and provide bonuses to all of them (the city where they were a leader would need a new one). For example, if farmer had a promotion "+1 food to the farm it is working on", then as City Leader this promotion changes and affect all farms in the city, and as Governor - all farms in cities under its control. When city achieve something important, for example build a wonder, create some unit/building for the first time in your civ, discover a new resource, gets majority religion, send or receive a new trade route, attracts Great Person, etc., the city leader gets experience and after accumulating enough could get promotions and get new abilities. Governors get expirience for achievements of every city under their control.
4) Religions - they aren't established by any civ, instead they just appear with random events, like "A strangers appeared on the outskirts of our city and preached about new religion. They managed to convince some of out citizens who had now adopted the Christianity."
Every citizen is affected by all religions in the city, based on believes of other citizens around it, and effects from the city, buildings, trade routes, etc. For example, the citizen in Christian and have +1 pressure for Christianity from itself, and there are 2 nearby citizens who are Buddhists, so they provide +2 pressure for Buddhism and after some turns the citizen would also convert to Buddhism. If citizen have Cleric occupation, then they have +1 pressure for their current religion. So in this example it would have +2 pressure for Christianity and +2 for Buddhism, so they wouldn't change their belief. Same for nearby Buddhist citizens, they get +1 for Buddhism from themselves, +1 from another Buddhist and +2 from Christian cleric, so they are all in stalemate and wouldn't change.
When the city have at least 3 citizens and more than half have the same belief it becomes the city's majority and adds +1 pressure to all citizens. Trade route to a city with a majority religion (the same one or different) adds +1 pressure towards that religion. When civ have at least 3 cities and more than half have the same majority religion it becomes civ's religion.
Each religion have an unique, preset core that couldn't be changed, but each civilization, after this religion becomes its majority, can add new aspects to it like with Policy Cards in Civ6 or maybe like with civic trees in Civ5, but it only affects this religion in their civ. For example, Russia and France both have Christianity as their religion. Russia adds "Celebacy" to it, but it only works in Russia, while Christianity in France don't have it. When there are multiple differences between versions of Christianity in different civs, they schism and become separate religions, so there is now, for example, an Orthodox Christianity in Russia and a Catholic Christianity in France. And then Catholicism spreads to England and becomes their majority religion, but they adopt some other believes, and it schisms again and now there are Orthodoxy, Catholicism and Protestantism.
5) Yields - Tiles have small yields, majority of yields come from citizens. Each citizen should have multiple yields, everyone from start have +1 gold and +1 production to reflect what they are working in the city and can buy things (so they provide workforce and support the economy). Depending on occupation and other stats citizens gets additional yields. For example, a farmer produces +1 food, and a tile the farm is placed on have 2 food, and the farm improvement itself give +1 food, so the city gets +4 food from this citizen working on this tile. When this farmer increases the level and gets promotions some of them may be to increase his yield or yield of the farm. Workers have +1 production, Merchants +1 gold, Clerics +1 faith, Scholars +1 science, Artists +1 culture. Others stats also affect yields, if citizen have some established believe, they get +1 faith. For each level of education they get +1 science, depending on their happiness and loyalty they get +1 culture.
6) Make hexes smaller, with city taking 7 hexes, while districts and improvements take only one hex, majority of wonders also take only one, but some may occupy more (for example, some wonders that in real life are built partially in mountains and partially outside of it, could in game be placed on 2 tiles - hill and adjacent mountain).
At the start of the game settlers could only create a barbarian camps on 1 hex, at first they could only work on tiles and produce units, but can't build anything, until they discover a technology and/or obtain resources and then upgrade camp into official city. For example, civ needs to cut the trees and accumulate 20 wood before they can upgrade the city, and after discovering masonry and having a stone quarry the city could be improved again.
Official city takes 7 hexes, and some of the buildings (and maybe wonders) could be only built on this hexes, and there are more possible buildings than hexes, so for each city player would need to chose what to build.
Maybe also further divide hexes into 6 triangles each and instead of the districts just place separate buildings into triangles, and the districts then form automatically if there are several buildings of the same type on tile. For example, you can build library and observatory on triangles of the same tile, and it then becomes Campus. But in another city there is a good place for library in one tile and for observatory on another, so they don't become a district and work separately.
7) Resources and improvements - luxury/bonus resources are no longer limited to certain tiles and strategic resources aren't discovered and revealed all other the map with certain technology. Instead there is a chance to discover new resource on every turn. For example, farmer could discover wheat, rice, potatoes, etc., and they then become available to be sowed on all other farms in the city, or they could domesticate cows and transform farm into pasture, or discover olives, wine, etc., and transform farm into plantation (and it would also change their occupation to merchant). Workers can discover new ore and turn ordinary mine/quarry into iron mine, or gold mine, or diamonds mine, or marble, or granite, etc., when working on forest tiles, they could discover some rare trees (luxuries like rosewood, bonuses like apple trees and strategic like rubber trees) and also turn them into plantations or they could discover animals like foxes, elephants, etc., and turn tile into camp.
Discovery of new resources can inspire the citizen to become a scholar or artist.
Every city have independent amount of resources (strategic, luxury, bonus, food surplus), and this amount depends on the locally available sources and amount gotten from trades. Exchange of resources within civilization is also done via trade routes, so, for example, city1 don't have a local source of iron, while city2 have it. The city1 can't train a swordsman, so you either need to send trade route from city2 to bring iron to city1, or train swordsman in city2. Ordinary mines/quarries and lumbers provide stones and wood strategic resources, necessary to build any building in the city and the city needs to have enough stone/wood (and later they also needs iron) in order to build any building and these could also be exchanged between cities by traders. Other cities in your civ can learn to grow luxury/bonus resourses by getting them from trade with your city that have this resource, or if a trader from your city to that city is scholar, it would learn about all available resouces. For example, city1 discovered wheat, but city2 don't. You can send trader from city1 to city2 with wheat, and this way city2 would also be able to sow it on their farms, but then city1 also discovered rice, while city2 discovered potatoes. You can create a new trader, or wait for the first one to return, before sending it again to deliver rice to to city2 and bring potatoes from it to city1, or you can create a new trader from scholar and send it to city2 with strategic resources instead and it would automatically teach city2 about rice and bring back knowledge about potatoes.
8) Combine units into armies with maximum of like 5-8 units. They move as one unit, but when battle starts it opens in a separate tactical map.
I mean it would be "realistic"...
