In an ideal world I would like to see many of the features from Vicky implemented in Hoi IV. From what I've read on HOI IV sofar I have hopes that some of it will be implemented to some degree atleast.
Factories: Represented on the map and require pops and input resources to run. Much larger resource variation to go with it. For example, airplane factory would require aluminium, electronics, lubricants and rubber together with a monetary investment. Individual factories can be designated with specific equipment production and require re-tooling to switch to a different type of production. More advanced products will require more/different input resources. Factories can be captured by foreign powers if their provinces are occupied. Perhaps chance based or at an increased cost. They can be razed aswell so the player could be forced with making choices as to keep em or burn em when an enemy is about to take a province. Historical flavoured factories could be implemented aswell, such as the Krupp works in the Ruhr and even be given an added bonus (such as increased output of artillery). Edit: Obv. these factories would be needed to produce supplies for your armed forces aswell.
Pops: Same as Vicky. Pops with their own needs, desires and political ideology. Can be influenced via the use of ... (forgot it's name but the thingy that is used in Vicky to influence them). To lessen the micro-management a bit and emphasize HOI IV combat focus, simplify their needs into one category: Consumer Goods. Also produced by factories. Would add many more layers to the game as the player has to keep pops happy, is given more insight&control to the political landscape of their country and has to balance their manpower between the military and industrial/consumer needs. Pops can also be used with regards to modelling partisan activity/GIE's.
More flavour, especially for the non-majors: Paradox games are as much roleplaying to me as they are strategy. Implement a generic general generator much like in Vicky. Make it optional to use if needed. Historically feasible names for newly created units. So, as an example, instead of 1st Inf Brigade use Infanterie-Regiment 1 for Germany. Let us see what type of unit we are fighting. More events, they don't always have to involve choices or modifiers aswell. Newspaper system such as the one in Vicky would be nice. Also the battle events introduced in one of the HOI III expansions was a cool idea, hopefully a bit better implemented this time around though.
More elegant difficulty settings: To the veteran player normal setting is probably too easy. The H and VH settings however bring with them their own problems. Such as boosting allied nations the same as your opponents and throwing the manpower/resource balance out of wack. Manually altering the modifiers in HOI III could relieve some of the issues but isn't an ideal situation. Hopefully they receive a little consideration in HOI IV.
I could list a thousand more things, instead I decided to control my urges and list a few that came to mind. I understand that factories & esp. Pop's might be a bridge too far when it comes to HOI IV, but they would offer an nearly endless source possibilities if implemented. Perhaps a simplified model could be used based on the same concept of Vicky.
And, as you can probably tell I'm part of the 'can't get complex and deep enough' school of thought.