Lots of people keep saying "but it's all good, everyone just went into debt and didn't have to pay it back until later". The thing is, though, they could only go into debt if there were sources to borrow from in the first place. This is the point I'm pushing - these things are important to a simulation of WW2.
That said, it doesn't all need to be about national accounting statistics. As per my previous post, IC pretty much did this (don't forget that how much money you got for international trade was what was left over after you'd met your consumer needs on the consumer slider, iirc (not guaranteed, set me straight if I've had a brain explosion) - so the rest of the economy was simulated). This worked, and did a reasonable job. It was a bit rough and ready, but by tweaking base IC at the start it can do a good enough job of providing a proxy for a combination of accumulated resources and capacity to produce (savings, human capital, factories, etc) and natural resources. By combining the two, it will fall apart over a longer-term game, but that's all good for HoI, it's only a decade or so (if we do want HoI to be extendable (as plausibly historical) into the cold war, then it's absolutely vital to take a more nuanced approach than IC).
I have no doubt that it will be simulated again, through IC or similar, and if it wasn't then there's no way HoI would provide anything like a grand strategy experience.
The thing that I find most perplexing is the view that countries didn't care about money during the war (or debt, for that matter). I get that we don't want to focus on managing tolerance of debt in a WW2 simulator, and I'm not suggesting for a second it be turned into an economic simulation, but that we at least maintain what's been undertaken previously. Even in the bounds of HoI3, if the consumer part of the economy was pushed too heavily (running below the consumer goods slider), then NU would deteriorate, and things went to pot. This is (roughly, but good enough for the game) modelling the impact of taking too much away from people and putting it into the war economy. Ie, we already have this modelled in HoI3 (and it's been a long time, but I'm pretty sure it was there in 1 and 2 as well) and it should be a no-brainer that the impact of money (through a proxy of IC, or through proportion of economic activity (in collars) for non-war/industrial consumption is the same thing, at the end of the day) will be in HoI4 as well.
But, remember, money is not just in there for the simulation. It needs to give the player interesting choices. In that respect, I don't think the primary use for money is really during the war. Instead, I think it's better used in the pre war period. As you said, for most countries, the prospect of invasion took off the brakes, the real rub was getting the needed money before the war started. Once the war is on, I think most players will want to be focused on the movement of troops and such and it would be better if the money management aspect became less constraining at this point. (of course, this needs to be managed so Italy can't prolong the war with Ethiopia to get a full war economy for 5 years)
As a gameplay mechanic, the goal of money would be to give the various European actors and America especially, more to do during the lead up to war.
That's the thing though, the impact of taxation/war bond raising/etc; do provide interesting choices. In HoI3 the decision about what kind of war footing you were on (what proportion of IC was consumer goods) was effectively how heavily you taxed/raised bonds, and it had modifiers for all these things. Further, how aggressive Germany was had an impact on how heavily its potential enemies could start raising debt and taxes to prepare for war. Ie, the decisions relating to money were already there in HoI3. As it's a grand strategy game, the choices will be there again for HoI4, in some form or other.
