Here's my following Suggestions:
Tiers of Production buildings, ranging from Proper Factories to Workshops, the former would be specialized into rough fields(say small arms, artillery, motorized, and tiers of Tank Factories for mils, and various sorts of civvy ones like Food Industries, Tractor, and civilian Automotive Industries), would be very IC for Slot efficient, but would be extremely expensive, and have the civvy ones give various buffs to your nation(this would additionally be an incentive to use the conversion feature as well) these would, however, have the downside of being permanently visible regardless of intel level and be relatively easy to bomb. This would also allow us to differenciate the effects between foci, for instance Greece deciding to make more Tobacco rn is about as impactful as kdf-Wagon, which was a key part of the building of the German Auto Industry that supplied the Wehrmacht during the war, and small "Workshops" representing smaller industry which would be cheap, but slot-ineffiecent, these would be hidden by intel but would be much lighter duty, though able to make the basics(See: Trucks, the basics in form of Infantry Equipment, Support Equipment, and Artillery, AA, and AT), there would be a IC Cost Cap for production of Armor and Aircraft(representing how though early armor and planes were decently simple enough to make while the heavier armor got and the better the planes got exponentially more complex, for instance) though these should still be plenty viable enough to convert and upgrade aircraft and armor. And preferably, something like triple or more the base armor capture rate, but have Armor become "Damaged" from battle and capture, requiring repair by some form of industry to become operational again, or able to just go strait to the conversions. Similarly I'd like to see planes be able to be damaged and incur a industry cost to repair, say when a plane gets hit but survives due to Air Defense, depending on the ratio of air attack to air defense, there'd be a chance that the plane returns damaged. This would also double to buff Speed and Agility to help it match with Air Defense
Apart from that, I'd like to see some capability to stockpile Resources, say for instance you can stockpile Steel or Rubber so that once the war starts, and you are prospectively cut off from significant additional sources of vital materials you can keep on running for a good amount of time, hopefully until you can secure a new source of the precious resources. Naturally you'd have to buy the resources to stockpile them, giving reason to increase the amount of resources bought in the early to mid game
And Finally the ability to adjust your Export Laws. Have the base forced Resource Export be reduced by half across the board, but have the potential buffs doubled or tripled, but be tied to the amount of resources exported. Have the basic buffs of the base forced exports, and then give the player the ability to either reduce exports of a resource, at the cost of reducing the buffs given from the law and potentially Civvy industry cost. Meanwhile exporting more than you need, say for instance YUG exporting more of its Chromium than it needs, or nearly all of it in other words. would increase the buff