I've thought that one way an economy could sort of be represented would be by mixing up how Military/Civilian factories are represented. It struck me as silly how military to civilian factory conversion is represented as more difficult than changing production lines, because something like infantry equipment could be turned into appliances or army trucks into civilian trucks more easily than the other way around.
So one idea I had was to retool it to where you've got Civilian Goods as a variety of types, and factory types are based on a broad class of goods - Auto Plant, Light Industry, Heavy Industry, etc. - so that it's easy to change between models of the same thing (like a newer warplane), more difficult to change the thing entirely (civilian airliner vs strat bombers vs tac bomber), and difficult to change the basic type (like planes to tanks). Add on some more resources like coal and food which are very important, but Paradox ignores food because mUh CiViLiAnS.
Then, have some thing going on where civilians have a Living Standard and it can be increased by all goods, but they get more if its in the proper ratio, like you can keep pouring in refrigerators at them but if they have a bunch of refrigerators another refrigerator will only slightly increase their Standard of Living compared to another radio. Can either keep it simple and have a single SoL for an entire pop type (like, ethnic group/social class), or have it be more complicated and have some way of representing that pops in overproduced goods don't get paid as much. All of this I'm talking about isn't an actual market. Sales of goods, whether to your own population or abroad, raise funds for the government. Then, the government can buy out factory space to make war machinery, with things like:
Exports and imports: Sell off your surplus goods, import scarce things
Wages and welfare: Spend money raising Pop Standard of Living, or artificially depress their wages in wartime
Profit controls/nationalization: How much tax does the factory raise for you
If the Pop has low Standards of Living, it will then be attracted to extremist/opposition ideologies, with specific content (since this is only 25 years, and only about one generation before historical WW2) being particularly to the nation; generally speaking, rural areas should spawn more Communists, advanced economies more Fascists. Add some shit like Millennium Dawn does where events come along where you can choose a few options with different effects/gambles that can effect things like your output levels or Standard of Living to phoney up a little economic system.
Broad idea of a 1922-1947 game: the world is already struggling from the rise of Communism and Fascism. During the first act things are going kind of fine - minor escalation, minor extremist activity, wealth - but then an exogenous shock, maybe, happens to represent a Great Depression (not good to have something like that be deterministic, but maybe necessary for representing this period over such a short time frame) which kicks off a much more violent and polarized second act, and eventually the revanchist tendencies of defeated powers and extremist activity leads to a world war, with the idea being you try to win the world for your ideology or just survive. If it had a slick diplomatic system you could try to build in some idea like a utopian, successful League of Nations run, but I don't know how you'd do that.
A grand strategy game, even if it tries to not be deterministic or project real history too much onto a setting, needs a story it tells, to say something about historical processes through its mechanics. Victoria II does that with the themes of industrialization and liberal democracy and, with PDM, a big showdown over the fate of the empires in the Great War. CK2 is more aimless but it still has an implicit goal in each game of reforming out of feudalism into a more Renaissance-style monarchy, or out of tribalism into civilized government. EU4 doesn't really have a point to it besides blobbing a colonial empire and going through a few historical events, and so it's far and away the weakest "story."
Basic difference between free market and command economy: have a Victoria 3 style Investment Pool where what you're allowed to build depends on your system, ie any given facility is public or privately owned and profits accrue to a public or private investment pool. The more capitalist you are, the more efficient your businesses are - in general, there's pretty much no economic benefits to planned economies - but obviously the capitalist pool is larger, which can only be spent on capitalist-owned things. If you want to rent out a privately owned factory to build things (like tanks), you have to pay for it. Capitalists are like boomers (big growth potential, but your resources are tied up in the short run), communists like rushers (mobilize resources faster) and also have more control/ability to respond to changes suddenly. Fascists are essentially just a hybrid, more akin to communists in practice with central planning so you might say that they have control over their factories but fascists are locked into more unequal income distributions and communists are locked into more equal ones. (You could also argue that Communism may have faster growth potential but at the expense of short-term population satisfaction since you can invest more, consume less.) Alternatively, if a person was trying to more explicitly base it around traditional rock-paper-scissors RTS and less on realism, then you might try to turn fascists, since they tended to support autarky, into some kind of turtlers, like they're not good at efficiency but they are good at maximizing raw resources out of their home turf (think German synthetic oil and rubber). I think that would be interesting, in an RTS it goes boomer -> turtler -> rusher -> boomer, which here would suggest fascists beat communists, communists beat capitalists, capitalists beat fascists in a one-on-one fight between similar sized nations? Probably getting way off in the weeds with this, but autarky bonuses would do a lot to make fascism distinctive vis a vis the scale-focused capitalists and the control-focused communists. A capitalist can achieve the largest economies overall, a communist can use their economy more quickly and maybe grow faster, and a fascist can get the most out of their land (naturally favoring sides which face harsh resource constraints).
Include debt, both to other nations' governments and to private sectors in nations. In emergencies (high mobilization laws) the government can do more extreme regulation, by the time WW2 is in full swing all the economic theory stuff pretty much becomes irrelevant, but its relevant to the build-up and the aftermath.
Printing money (inflating the currency) would be a thing you do that temporarily reduces the standard of living of the pop to generate more money (and if you represent different nations as having different currencies, which could actually be really important for representing the situation of Weimar Germany and the early Soviet Union), and if you inflate to very high levels you start to get a chance of hyperinflation triggering which crashes the standard of living, shutters a bunch of your businesses (low efficiency, or shut them down and have to pay to reopen, something like that), and voids all of your currency.
