OK, here´s my two pence worth. When it comes to production, I´d really really really like to see the economy model expanded to cover the national economies in a more wholesome fashion. For example, I´d like to see taxation, budget etc. I´ll point to the direction of games like Supreme Ruler series and the unfortunately cancelled East vs West, which had a GDP value in every province, at least from what I gathered from the screenshots. The underlying reason I´d like to see these things is that I propose to make a mod (unless Paradox plans an official DLC/expansion) that expands the timeline possibly to 1900 (WW-1) - 2050 (Cold War and modern conflicts like the ongoing Ukrainian unrest/conflict). For this to work, the game really would need a more in-depth economic system.
As for resources, I´d request if it at all is possible, that the resources (tungsten, iron, coal etc.) be pushed to a text file so modders can add new resources. This would pretty much solve the problems for expandability I guess, since taxation/gdp could be added as a resource, same as agriculture/food (required by pop and can also be converted to alcohol fuel), fresh water (Supreme Ruler series has this and future conflicts may very well be for this resource).
Of course adding new resources also means adding the functionalities for them, like which buildings require them and how much (say light IC or small arms industry, 1 iron 1 wood 1 electricity and 0.5 money per day coded in the buildings file as daily resource costs.
Manpower could be calculated from province population with a formula, say that a province has 500.000 ppl (Helsinki for example), manpower is simply a percentage of this number calculated by using different parameters like conscription age set in conscription laws (say, men only, 18-60 of age, 30% of 500.000 gives a manpower number of 150 units of 1000 men total. that as a rough example). Also taxation could be calculated from population (per capita GDP). Additionally, let´s not forget that industry needs manpower as well to work, if you conscript all your farm/factory workers to the armed forces you must also organize a womens auxiliary army to fill the gaps in labor force, otherwise your industry will grind to a halt and your citizens will starve creating dissent which will soon boil over into a revolution.
To conclude, I´ll quote an old Roman proverb; "The sinews of war is a bottomless purse". In order to win you must have the economic/financial resources to do so.
Cheers!
EDIT: I´ll weigh in a bit on the infantry equipment discussion. Deciding what type of a standard weapon your infantry wields is indeed crucial in my opinion. Bolt-action rifles are simpler (more reliable) and cheaper to produce than semi-auto rifles (at least the early models had problems with reliability if I remember correctly) or assault rifles, but they cannot stand up to the sheer firepower of automatic weapons in a firefight. What type of a weapon is most effective depends on several factors as mentioned, terrain, vegetation, urbanization etc. Personally I view this from the peculiar standpoint common to northern latitudes (Scandinavia, Finland, Alaska etc.).
What type of a weapon will work and keep working in freezing temperatures where the WW-2 era gun oils etc. would freeze rendering the early automatic/semi-automatic weapons unreliable? Now the Russians did an outstanding job in designing the AK-47 on which the modern Finnish army rifles are based, just imagine if you rushed ahead in design and were able to equip your Finnish army with assault rifles for the Winter War? A flight of fantasy yes, but just to illustrate the point that it would have the effect of doubling or tripling the firepower of an infantry unit, the assault rifle indeed was a revolution in firearms design and in my opinion the game should reflect that in some way.
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