A few suggestions, as I keep playing the game.
1. Tribal Villages
a) I think tribal villages should have a small chance to grant a great person (including great generals). Most of the times, especially early game, it would be a huge risk to go back to the city to settle so they'd likely be used to bulb, which is slightly better than the already granted free tech. Though it might incentivize sending out a stronger unit to claim tribal villages so they don't get snagged by barbarians on the way back to settle.
b) Hostile tribal villages should also be slightly more dangerous, with the units in the medieval and later eras not being primitive warriors, as well as regional based. Say in the British Isles they'd be Celtic Warriors instead, Strength 4 or 5 instead of 6. This could be enough to keep Ireland and even Scotland (CW takes settler) independent until conquered by Vikings or England.
c) Touching a tribal village has the chance to spawn a barbarian city, with your unit being immediately yeeted out of its borders. You have the chance to conquer it unless its in a core, historic, or contested territory, it immediately gets razed. However if you let it be its either the next guy's problem or it gets flipped on a spawn. This could merely lead to less prominently settled city locations being a pillar in this game.
2. Resources and Stock Exchanges
Resources always annoyed me in Civ in a few ways but so far DoC tackles one of the main issues with the "limited resources effects." Stock Exchanges could be adjusted so when they're built you get a quest like harbor master where you found a Company for a specific resource. This is not an industry, but a task to gain a certain quantity of said resources. Take banana for example, with the quest "United Fruit Co", X banana resources within a certain amount of turns grants you a great merchant, and every city with banana in its BFC gets a free merchant. This will incentivize expansion wars with a deadline for a huge reward.
3. Running Slavery with Free enterprise should add an extra +1 gold for Farms, Mines, Orchards, Paddy Fields, Plantations, Slave Plantations, Quarries, and Workshops while also giving a +1 trade route per coastal city. However, it should also have more diplomatic and stability penalties, as well as common negative random events.
The purpose would be to give players the choice to risk collapse but make a lot of extra money in the meantime before entirely toggling off slavery... or collapsing.
4. Disappearing beaver
Beaver should disappear in this game just as how many resources appear. For one, there could be more beaver in the New World between 1600 and 1800 before disappearing, demanding the land be repurposed and of course less profitable afterwards. This will make British, French, and Dutch colonies that will be lost to the US and and Canadian regardless more worth the colonization efforts beyond just completing a UHV while not over powering the land by modern day.
5. Iron Overhaul
Iron is weird. Most armies had as much as they really needed in medieval times but by the industrial age there were only a few places world wide that had sufficient iron deposits. In one case it was right on the border of the biggest rivals in history, Northwest Alsace Lorraine and Longwy Brie between Germany and a France.
In this game we give everybody that historically had an armor suited army an iron ore deposit. Instead I think we should take most iron ores away, but not taking away the ability to produce units. Instead controlling an ore deposit gives certain bonuses. Say a swordsman or other iron necessary unit has a +2 coin cost per turn that gets neutralized by access to iron. This will allow countries with access to iron ore be able to field bigger armies, while still allowing countries without it the ability to defend themselves, but maybe not go on the offensive exactly.
Access to iron would also make constructing railways take 25% shorter and adds 25% production bonus to tanks and artillery.
With this improvement in mind we would drastically scale back the iron resources in game and put most of them in contested territory, or at the very least code many to disappear either based on era or triggered by a collapse. The Byzantines collapsing for example would remove the iron in Georgia, whereas the iron in Persia would disappear after the collapse of the Seljuks.
All iron removed from Europe except in Spain, England, Ukraine, and Sweden. That means Portugal, Italy, and Greece would lose their iron. There is one iron in eastern France and one in Western Germany that would be combined into one ore and moved 1N from the hill iron is currently on NW of the Alps. Poland would swap just the iron and the coal. There would be an additional iron added 1W of Stockholm or replacing the uranium to give the Swedes an additional iron to trade away. The Spanish iron would disappear with Firearms or if they collapse in the case the Moors take them out. Austria's coal 1E of the mountains would be an iron that would flip to coal with the discover of geology or collapse.
These changes would encourage war between France and Germany and Germany and whoever controls Krakow. It would also make powers vie to trade with Sweden for their second iron.
In North America there'd be three iron resources. The one in the Appalachians would be removed and replaced with stone, with the stone there replaced with another coal. The Quebec one and the one in Mexico also removed. Instead the iron would be 1. On the grassy hill north of lake Superior, the hill two South of the in Minnesota/Michigan's UP, and one right between the three cotton resources, which is an area missing production resources anyway.
This gives America two iron resources with incentive to conquer the third in Canada. Mexico, again, can still train a medieval army but it's costlier and they tend to skip that phase anyway.
6. Borders
a) On the continent of Europe the discovery of Statecraft allows tiles not to change hands with a civilization you are at peace with regardless of culture.
b) I have no idea how difficult this would be but tile negotiation. When making peace you can request specific tiles with the BFC of any of your cities. There's two ways I'm thinking this could work. One is triggering the advanced start function and having peace treaties go through an interface like that. Or a function of diplomacy where you contact a civilization and instead of peace treaty 10 turns and cease fire you also get an option for an Arbitrator.
When you do that a unit called Arbitrator 1 spawns in your capital and you can move it like you do a plane to a *visible* enemy tile in your BFC and send them there. It cannot attack, can't go to a city, can go on water, and only has the visibility of the tile that they are on. Once they do go to that tile they disappear and Arbitrator 2 spawns in your capital and you send them to a different tile, with the previous tile being inaccessible. So then once you do make peace one of the terms is land area and it's numbered. So say you sent 6 arbitrators in and 2,4, and 6 are redded out but 1,3, and 5 and white, you are allowed to negotiate on those three. Again, I'm sure this is probably difficult AF, though would be very nice.
c) City Occupation. In warfare I like stomping out the enemy but I don't like being left with their cities if they collapse or capitulate. Could an option be merely be "Temporary Military Occupation" where the city is under riot but cannot produce anything and inflicts the same amount of war weariness. When peace happens the city automatically reverts to the other party or in the event of collapse besieging forces are sent on the outskirts of the city.
7. Minor stuff.
Before America founds its first city the name should be "American Colonists" instead of "American Peoples."
The Lake representing the Great Salt Lake should be moved up one, the Vegas tile should be moved one south, that city should be named St George and settler score negligible. The Lake would be SLC and a plains tile, with 1E also a plains tile with stones on it. That way we can actually get a decent city there.
Oil should be removed from the Denver region and instead places 2N of Washington.
So far that's all.