ok. so this thread got moved and will now die instead of contributing with a wider discussion around ideas.
I may not be the Boris Gudenuf Winger Hussars charging in to save the thread, but I was thinking about this all day!
While I won't try to give a magnum opus post, I will throw out 3 things that i seem to be drawn to when pondering the 4X games i play.
#1: Victory and Tension
Most victories in 4X are positional - IE, do something first or be the best at it. Score, conquest, etc. That positional aspect is the key, because the closer the empires are the more tension the game has and generally the more fun players have. Personally, I think the decision to formalize eras in Rise & Fall, and make it affect game play, was one of the
potentially most impactful changes made to the series in a while. I think that making each era something of a minigame of competition between players is the path forward for this. For example, instead of just getting golden/dark ages from era score, players would be competing in various categories, like most cities converted, most techs researched, most wonders built. These would shift over time, but each new era would form a sort of blank slate where even civs that haven't done so well can suddenly shoot up in a niche that the leading powers aren't really focusing on. (With my sort of cost balancing, imagine a player who fell behind in tech in one era catching up: it's easier to "improve" than to further your lead.)
I would broadly have the game balanced so that you cannot truly have it all; imagine allocating your focus between several categories - say, [trade, science, culture/tourism, religion, military] and I give you 5 "points" of focus. You can put one in each category and have a balanced playstyle, you can neglect one for to double up in another, etc. This would generally subject the player to diminishing returns, but it would allow a lot of options on how to run things. A korea might go all in on science at the expense of the military; another civ may really push the military and use conquest to make up for a lack of trade. Putting all your eggs in one basket won't allow for the same effect as science spam does now, where it means you can win in all categories. To do this, I would impose a sort of "arctangent scaling" on research rate. It is regular effort to research things contemporary with the world era. It is progressively more difficult- becoming exceptionally so- to research things ahead of the world era, and progressively easier - becoming exceptionally so- to research things behind the world era. This modifier affects your research rate, but it's monotonic: the player with the most science will always have the lead, the player with less will always be behind. The difference is you don't get these crazy 3 era gap blowouts developing, and any empire can focus themselves to "catch up-" but this does not necessarily mean
overtake!
#2: Economics
One thing gamemakers always need to contend with is how detailed to make the game. That's a judgement call. I enjoy some detail but complexity for complexity sake isn't something I prize.
Economics
As far as economics go, I think representing the key metrics by the three yields - food, production, and gold - is a very good and classic mechanic that many games (Age of Empires, Civ, Stellaris) have in some form or another.
I think there is a place for resources, but the perfect game, to me, is one a lot of other people would play too, so having lots of schemes of resources on the map->manufactured resources->final product is probably out. But I'll get into that in a second...
Tying in with the central focus of the era system in #1, the basis of the game economy should be forced to change era to era. For example, a slow shift in production coming from terrain vs industrial infrastructure. Etc.
My largest civ6 gripe is how common gold is. I think the ease of generating it removes a lot of empire management opportunities. When I discuss economies changing over time, I recognize that part of the incentive of getting a new era's unlocks is that it helps you solve the problems you currently have. That said, I think it should also introduce new issues. Using the principle of consistent rate of return, I think a
critical lever of game balance is to make financing your empire reflect the military strength system in civ6: you can make more money over time, but you need it to pay for more expensive stuff. Let's just say that, for example, in an empire that was fairly balanced, it might spend 10 gold for every 11 it produces. This margin of (example) 10% is a number that should be pretty consistent over time: whether you make 10 gold, 100, or 10,000, you will be spending it too. You won;t end up with civ6's late game where maintenance never escalates and you make +500gpt and don;t think about it for several eras at a time.
The reason I like this is it means we can keep our carrot and stick tech incentive. In civ6 terms, say I currently have an empire funded by my market buildings. I don't have a lot of surplus gold income with which to buy units, conduct diplomacy, pour extra into other areas. But I am about to get banks! Banks mean I can suddenly fix my budget and afford working towards other areas like science! But... I will also unlock universities, and opera houses... which I need the gold generation of banks if I want to pay for them. Now I'm back to where I started and I am actively looking forward to stock exchanges.
I wouldn't have things so spaced out, but you get the picture of a "tick-tock" process that first gives the means to solve existing problems, then gives you new problems/opportunities. The same concept applies to production - more productive infrastructure is needed to produce everything you want to build, but also will be required because all the new stuff costs more. Etc.
In order to break the wide/tall extremes, i would make the economy ultimately centered around pops + infrastructure/policies for them. In civ6 terms this would mean that a campus full of scientists will really dominate a campus without them.
Resources... As far as strategic resources go, I think keeping the game limited to just one level of resource production is ideal. As an example, see stellaris: much more complex economy, but even they only have the three basics of food/minerals/energy, and the manufactured/acquired alloys+consumer goods+strategic resources. Military units need alloys and those rare resources.
How would I do it? Let us suppose we are staying somewhat true to civ in terms of resources for a moment. So an early game resource is iron. You can find iron on the map starting in the classical era. You need it for swordsmen. Having an iron mine is a great thing, but we won't totally hose you this time: you will later unlock the Forge building which allows you to run a project to
make 20 iron for X production and drop it into your stockpile. The ability to see and mine iron would come first, and would serve as an advantage. But anyone can make iron for a cost. This would be paired with the military: at first only swordsmen need that iron. Between the classical and medieval you get the forge. In the medieval, more units want iron - knights and possibly foot soldiers. This would mean that unless you are blessed by the map generator, even players with iron mines will likely need to run iron projects to keep their military modern. Niter is similar - you first see it in the renaissance, but then you learn to make it, and between renaissance and industrial a lot more units want it. I would apply something similar to aluminum + horses, and any other material resources. I'm not sure if this should be extended to fuel resources like oil - chiefly oil. I would have a global market mechanism later in the game to allow players to pay gold to buy more for their stockpile regardless of diplomacy, (see how I just gave a path for production focused players to produce resources to sell on the market?) but synthesizing the stuff would be much more difficult and resource intensive - something you really only want to do in a pinch. And, these would exist because the world congress might be able to ban players from the world market for bad behavior.
I would also make resources a benefit you can take advantage of, but don't have to: there are always military options that don't need resources, but those that do are superior. See infantry vs tanks or pikemen vs civ6 style longswordmen. (Assume combat was balanced such that it would be possible to do such things!) Similarly, the economy could also consume resources. The Power system is obvious, but infrastructure might enjoy it too: I made a personal mod to add mutually exclusive factory alternatives that consume strategics, namely the Steelworks (eats iron, but +2 prod to all districts in the city) and a fertilizer/chemical plant (eats niter, but +production to farms, pastures, camps, plantations, etc in the city.) Conceptually, we can have more advanced structures that consume a resource to provide more of a basic yield, or we can have things like a Steelworks that eats power and coal, and lets you more efficiently make iron than a Forge would. Helpful UI tools, like automatic world market trades, or being able to run a resource synthesizing project indefinitely, would avoid micromanagement hell. A lot of buildings would require power like Civ6, but this would become much more pronounced than now - for example, once you have a research lab, each scientist working in the city will consume power but provide more science. Ultimately, the economy is something a player must invest in, and
can choose how much to invest in, rather than static infrastructure that you fabricate and forget about.
To complement the role of strategic resources being used this way, I would like to see luxury and bonus resources become something more impactful over time. Since strategic resources are mostly serving as a
concentration or
abundance of something instead of their literal presence/non presence, the core of what bonus and luxury resources actually do would be similar. It's nice to have them, but you can get the results - food, prod, gold, amenities - other ways that just take more effort. First of all, luxuries would give a diminishing effect for multiple copies - like provide 4/3/2/1 amenities for the 1st/2nd/3rd/4th copy you have, etc. Second, bonus resources would unlock special buildings to boost them and other things, like how the forge, stable, and stoneworks of civ5 worked. (This is an anti-chopping incentive.) But, in a historical 4X context, luxury resources would play a key role in 1) trade routes 2) the colonization phase of the game 3) the industrialization phase via corporations. This is to give players a reason to want them even if they can get amenities other ways. I've suggested this before, but what corporations would loosely do is consume one of a few target resources for a primary yield benefit. The luxes would be split into a few categories, and each player could only choose one corporation (but it may be the same as someone elses.) The corporation mechanic acts as your tool to economic influence on the world economy.
Corporate benefits would affect physical control of a resource itself, getting it traded to you by someone, as well as trade routes. Like religion, its an optional system. It may even have some customization ability just like them! They don't show up until around the industrial era, though.*
*Not that we can't have an ability to get one a little early, EITC style.
#3: Empire Mgmt
So this one is hard because the exact implementation is difficult, but since I am dreaming i will be a little wild. Basically, while your economy feeds the rest of your empire, you do need to keep things in balance. Funneling extra resources towards your production capacity or your gold income will mean less available for your research efforts or your military. And this isn't just resources: because I want the primary balance of empires to be around pops, the entire concept of keeping them supplied with amenities becomes very central. To this end, we need some methods to differentiate what empires are actively doing for their people and up the stakes over time; and I think the best way to do this is to borrow the terminology from Stellaris: living standards. People talk about this concept a lot, and I think that independent on what policy cards (or civics or social policy trees or whatever we call them) you run, you have a couple basic decisions as a government you can set. This one would be the living standard across your empire. Replacing the "happy/ecstatic/we love the king day" of present games, essentially the living standard is a choice that modifies how much amenities your people consume and how much they can produce.
Rough idea would be the ancient era starts with a "basic subsistence" and over time you get new options, eventually old ones phasing out. Progressively more advanced standards slowly raise the productivity of your empire (like ecstatic does now,) with end game, futuristic standards like "utopian abundance" being significantly more efficient than basic subsistence. This lever can also affect loyalty (if you are above or behind your neighbors) and per pop outputs - while I would actually do away with innate science/culture, i would bring in gold generation per citizen as a standard.
Certain government types would enable special choices that affect your people in other ways. For example, the ideology governments might allow for an extra choice that would have trade offs (imagining some requiring less amenities than other standards at the same level; or giving different empire level effects.) The most advanced standards could even start creating per pop power consumption in exchange for the greatest benefits.
Over time this might mean pops go from costing 0.25 amenities each, to 0.5, 0.75, 1, and maybe some late game choices exceeding that. I do envision specialists that just produce amenities for our entertainment complexes etc, and some way to basically use gold or tie up city production to supply more amenities than even those specialty infrastructure set ups allow. (Like bread and circuses but formalized.)
The result is that you could have a research kingdom where basically everything is tied up supporting your nation of PhDs; or perhaps you need to run a peg below the average in order to cover the needs of your conquest gains. But not delivering on the promised quality of life would have
huge implications, to the extent that your empire can start falling apart until the remnant can be managed. You then have a choice of whether you dump your economic output into your pops, or if you use it to further other ends - because you only have so much "guns and butter" to both supply existing pops
and keep your empire growing/expanding/building up.
This whole post is more a few theoretical frameworks in civ6 language that I think would make good principles to build a full game around
that could be civ7 or some other thing.
per level bonus, up to 5 levels. So while changing a unit line would wipe out its promotions, it would retain veterancy.
and 
and resource hungry. End game balancing is such that the last upgrade for several unit lines - Mech Inf, Modern Armor, Rocket Arty, for land units - are intended to serve alongside the modern era troops. They are stronger, but also consume more resources and fuel in particular. So you'll want to maintain as many advanced units as you can afford, and fill in the rest with "regular" units. Recall that the tech tree has upgrades to help units stay relevant when they have gaps - crossbowmen in the renaissance, spears in the classical, etc. Especially true for ships - once you can make quads, galleys get improved hulls too so they become true contemporaries.
