Okay, so what does all that stuff I said look like in practice? Well, it's hard to say. It was easy, Paradox would have done it (because easy is the friend of lazy).
Mostly I wanted a deeper economy and trade system, something that can really be played with instead of just building up my Trade button. Even Rome 2: Total War has more detail, in that you can stack wealth of certain types and those types are then effected by different modifiers (a partition of economic activity into commerce, industry, and agriculture). My biggest idea was having holdings be called Settlements and have counties have land resources that are divided up among the Settlements (generic holdings) as well as an abstract population figure (like Imperator pops, perhaps). The pops are self-sufficient and based on farming efficiency produce surpluses, rural craftsmen, and a pool of surplus population for the Settlements. This is all like MEIOU and Taxes so far. Let's take some inspiration from the Solow model and have a capital investment progress bar that fills up to some maximum. Raiding and civil strife depletes it, mass death from plague actually raises it (it's relative to population). Rural population can also specialize in some kind of noted craft that produces trade goods, like French wine, Italian olive oil, Cheddar cheese in Britain, almond milk in Egypt (no joke, that was a real thing that was a huge export to Medieval Europe), sugar in Mesopotamia, iron in Scandinavia, timber in Russia, etc. Developing these sort of industries is the kind of thing the settlement owner can chuck cash at
You get enough industry going in an area, you can maybe start establishing a proper sector. In this time period I doubt that tools, physical capital, are really the constraint. It's probably the labor, the craftsmen, that are, and so let's keep that character focus and have it revolve around guilds. The guilds are like characters unto themselves. We don't expect most Settlements, maybe even most counties to have one. They reflect a specific industry, have a reputation that can wax or wane. They're a source of events, can be political actors in ways. Basically it's like a character to interact with, but representing a group of people. They produce your industrial export goods. Leather. Armor. Cloth. Books. Candles. Carpentry. Whatever it is.
How does trade work? I don't know. Probably have some notion of trade capacity, trade routes you can set up. I kind of like the idea of having feudal lords be inherently crippled at doing this, really only able to link up between bordering regions, but the urban governments beneath them and merchant republics independent of them are potent at it. Make it require some sort of bilateral exchange. Paradox games tend to miss this, trade isn't a competition between you and the customer, it's a COOPERATIVE EXERCISE. The value of goods should reflect some sort of ideal balance, if you glut a market where too much of something it should have a very low price, generate little wealth, as a consequence.
The big thing here is that your economic system can always take care of itself, or at least to some extent. Maybe you can overdevelop if you get an import market in food going where you're dependent on the food imports, especially for massive cities like Baghdad and Constantinople, but in general you don't have to micromanage things like Victoria because that's not the main point of a game like this. But you do invest in developing your trade goods (through rural estates and through urban guilds) and your trade routes and that's the main work of it.
Socially, I've had this idea of counties having religious authorities (again, like another character) and different social classes with different attitudes. At a basic level it's the peasantry, burghers and gentry (lesser nobility below the level of barons and their equivalents). These are not proper characters, are not really active actors, but they may have some interactions, occasionally spawn events and such, most importantly they have opinions and can individually be angered into action. The local religious authorities SHOULD be characters. I don't like how CK2 represented all priestly characters as landholders, the prince-bishopric thing is only a Catholic thing and it forced it on the whole world. I don't like the idea of there being "temple" holdings. No, there may be cathedrals and monasteries and such and they may have characters attached to them, but in general, a county has its top religious cleric. Depending on the local attitude towards religion and your relationship to the cleric, they either act as an assistant or an obstructionist to the regime (religious institutions basically WERE the bureaucracy back then, a bureaucracy shared across borders) and as a propaganda mouthpiece for or against the ruler. At the same time, under conditions of low Moral Authority an antagonistic relationship between cleric and ruler may actually please the public. The big thing here is that the clerics are powerful even aside from the Pope. Even without prince-bishops (who should still be a thing where accurate, Settlements with a theocratic form of government) the religious community should be something the ruler has to constantly grapple with.
When social classes get angry, it manifests in civil disobedience (of the types they had back then), banditry, guerilla warfare, and ultimately open revolt. In this time period peasant rebellions were usually crushed flat, it's mostly a penalty to the economy/viable levies. Something that saps the ruler's strength like a debilitating disease rather than a sharp, pointed threat like a modern revolution.
Some Settlements may have a sort of settlement-within-a-settlement, basically buildings that have their own branches and a name, potentially even a character tied to it. Think things like cathedrals, universities, and monasteries (which does include crusader orders too, like Knights Templar properties). All three of those should be a big deal, very prestigious, very competitive projects that develop local economies, raise piety, raise the status of the church, solidify the ruler's grasp, and are developed over ages. In a post above I described the idea of pilgrimage as analogous to tourism depicted in other games. You investing in your religious/ideological infrastructure increases the strength/value of that religion/ideology to you. It is both cooperative (everyone is building the Church) and competitive (the ones who are better at building have more to gain, over their share). In general, universities are the capital to produce even more religious human capital (more scholars), monasteries are producers of fine goods and R&D labs, more or less, and small pilgrimage sites. Cathedrals are the big boys of religious tourism and religious propaganda. Constructing these fuckers should feel epic, like a true achievement that you love your cathedral when you are done with it. The game should be balanced so that any reasonably sized realm has several major ones by the end of the game, but they are a grind to get through. I wouldn't mind the game revolving around them like wonder races in Age of Empires.
Foreigner's quarters - Jewish ghettos namely, but also the merchant republic trading posts, the Hanseatic quarters, protected minorities of various types - also have such buildings, and grow by themselves over time. (CK+ had this already, in a very simple form, if a Jewish Community is left alone it eventually levels up and you can't build them. When expelled it triggers events for other rulers to let them in.) Jews and colonial Germans are treated like guilds, basically, a special type of guild. Jews should be rather prominent. Credit should be something that is always available (people always circumvented religious laws), but the more Jews and Templars you have the more credit you can get. Monastic orders should be playable as a way to play as theocracies. In a game about dynasties, monastic orders are the religious equivalent of dynasties (for Catholics), with the goal being angling to control the Papacy. You can be the state within the state, owning Monasteries, controlling Cathedrals and Universities, and sometimes owning worldly properties (like the Crusader States in the Middle East and Baltic) across Europe. Gaining a nobleman's child as a member of your monastic order is the equivalent of political marriage.
For Eastern Rome, their bureaucratic govt is reflected in a playstyle where dynasties depend on their rural estates (a settlement type that is like a castle but less militarized and more commercial). Just because you don't hold offices, that doesn't mean you don't exist as a powerbroker. (It does mean you're probably a shitty powerbroker and are going to have trouble holding on to your private properties.) For merchant republics it's about their private fleets and their trading quarters (CK2 already did a good job with that, but it fucked republics up in other ways). This would apply to China if included, just noting that in the Byzantines they were more of a martial elite whereas the Chinese literati/gentleman-scholars were an intellectual/academic elite. For any sort of classical republic I think dynasty works fine; for peasant republics, probably dynastic but portrayed more like a clan.
Settlements will have a legal system that reflects what "type" it is. This is malleable, it guides how the settlement may develop, and there is a memory of past traditions; try to revoke republican rule in a city and the locals will be angry for a long time, for example. In general, you can have your castles and your cities. Castles reward . Rulers will also have a docket of laws, decrees, things like that which effect the status of the social classes. Sumptuary laws, for example, may raise Piety/Moral Authority, please the clergy and the gentry, but anger the burghers. Droit de seigneur (I know it was made up, but I'm just using it as an example) pleases the gentry but angers the peasantry greatly. Balancing act stuff like this, much of which effects economies/levies/religious institutoins/etc in various ways. You are a RULER. You RULE. You pass laws. Petty shit is reflected more in abstract laws governing things like how heavily things are regulated. But in general, I want to be able to actually legislate what my filthy serfs do.
Nomads should act like in Attila Total War, they're settlements that literally move around with their armies. At least if they want to. They draw wealth more from herdable animal stocks that can grow or shrink but do require access to good land nearby, not just in their "county" but many counties around. The steppe is an empty place, mostly just so much pasture. CK2 implemented nomads horribly; they should be able to be multicultural, have different cultures/religions within the camp. The Islamic world should have nomads and hybrid systems of government scattered throughout it. There should be areas that are effectively off limits to everyone except nomads. I don't mind having the Sahara contain areas that are effectively impassable except by camelry forces, you want to conquer across the Sahara or maintain an oasis archipelago, play as desert nomads, otherwise, fuck you. I don't care if the map can be world conquered, if it can the devs failed to balance the game properly.
Navies should be like Attila Total War, in the sense that they're an extension of land combat that still allows for purpose built specialized fleets. When you load up your fleets the ships change to that "type," like an archer ship, heavy cavalry ship, etc. Naval combat has Volley, Ramming and Boarding phases and naval travel is confined to coastal provinces. Norse are an exception with the ability to transit through deep ocean, perhaps even getting exclusive access to Iceland, Greenland and Vinland (this also requires colonization mechanics, but that doesn't have to be detailed) in that way. Merchant republics and Byzantines get special naval units.
Rulers are peripatetic in Europe. You don't necessarily have a capital. You have a court and the court can move around, court may have its own buildings/assets but it's not the same thing as a capital, it just is wherever it is.
I don't remember what all else. Why the fuck did I spend so much time writing this.
P.S. Mercenaries should basically be a special type of nomad. It turns out (I had to look it up some time back) that they just kind of wandered around and tried to intimidate whoever they could when not actively fighting under a contract. Makes sense for them to be like these parasites that dynamically emerge, especially around war, and wander Europe. Don't give a shit if they're playable. I think you could say that if a mercenary captain dies without leaving anything behind he kind of failed as a warlord.
Parliaments should also be in, like Magna Carta type stuff. The Council was always too easy to manipulate but on paper I loved it. A Parliament would be a great way to represent a different type of development, you cut out the greater nobility's stranglehold but the lesser nobility - the barons and the gentry - get to step up and play a role in actively changing laws. I used to love the old Crown Authority system, yeah on paper it was worse, but in practice it felt thrilling to pass a new CA level and the fact that the whole realm got to vote meant it felt like a real act of legislation with having to rally votes to my side on a massive scale.
