I think we should get all the good stuff from Order of Battle in detail:

1. Frontline System and new supply/core unit rules
Instead of having a unit limit per map we should have a supply limit per map. Now every unit needs a certain supply amount per turn therefore the unit cap depends on the units used in a map and how well you can maintain supply support from cities or supply trucks. This also means I have the choice to go with 20 infantry or maybe 10 tanks before my supply limit is reached. It gives far more freedom to place armies and is much more realistic.
Also, supply can only be traced to your frontline. Once units get encircled they dont receive supply anymore and lose morale and fight worse.

2. Strategic Campaign
We should have a real campaign map like in Panzer General or Order of Battle where I can pick my battles and each of these offer a certain reward. Lets say I conquer Swiss, then my reward could be access to better mountain infantry. Or I conquer Berlin and I find the plans to V1 weapons, or I conquer XY and find a new MG so that all Infantry gets +1 soft attack. Campaigns but not only should give small one battle bonuses. Let me decide when to attack a country. Maybe I want to attack russia later with my German troops. But then Russia gets +10% prestige more per battle after 1941 or so. TO sum up, give Panzer Corps a strategic layer on a campaign map next to the battle scenarios.

3. Special abilities and customization
Give certain units special abilities. For example let pioneers construct trenches with a click (cost x prestige) or let them lay/clear mines. Let us camoflage our tanks and airplanes before a battle to give them a tiny bonus in defense/attack on certain fitting terrains. Let us level up officiers in certain units and unlock special abilities or other boni. Let us rearm airplanes. There is a lot from Torpedo to HE bombs we could do. Or let us give pilots a certain training (maybe even remove a xp pilot from combat and all new pilots start with +X xp for he is training them).