One thing I don't like is the lack of "attrition" for units on the front line. When units are on the front line, even if you dont initiate attacks, they should suffer causualties and organisation loss or regaining organization more slowly. Also, the amount of units on a province should affect the supply effeciency of each unit. Units should move more slowly if they are next to the enemy because they have to mantain battle formation. Units moving perpendicular to fighting should automatically suffer a 25% flanking penalty for a few turns. I really don't like how reserves are meaningless in this game and you can simply reinforce from adjacent provinces.
Units acheiving a breathough should move faster into enemy provinces than normally. Land battles hould alos be conducted more like naval battles with the idea of distance determining causualty levels and organiztion loss. With the ability to choose the battalion composition of a divsion (which i think is happening) artillery should get a HUGE boost in importance (don't confuse that with improvement in strength). Also, I don't like how units in retreat don't suffer causualties. If a unit is to slow in retreat then faster attacking units should do damage to it but I'm not sure exactly how it should be determined.
HOI looks at war as a science, not an art. War is not just about quanitative values. Morale, experience, organization should influence battles more than they do now. The research tree of land doctrine is too much set in stone. It should be more fluid and flexible and play more of role in determining battles. Why does Kampgruppen or any land doctrine have a specific research year? Land doctrine isn't some technological breakthrough, its a conscious desicion made by the higher ups of how to conduct the art of war. Doctrine should influence the use the three arms of the land combat: tanks, infantry, and artillery (in terms of battalions). In fact, it would make more sense to have a competely new page called "land doctrine" which has a bunch of different sliders (like 40). For example, there should be slider for movement vs. firepower, having consequences in the effectivensess of artillery, the amount of casualties suffered, combat speed etc. However, in order to preserve the "art of war" the effects of each slider should not be given. Also, the one year wait for diplomatic sliders doesnt' exist. Other sliders would be more specific like "armoured unit doctrine" with tanks on one end and infantry on the other, which affects attack, defense, combined arms bonus, and etc. The whole point is to give the player the ability to design military doctrine to his liking. ONe thing that has to be avoided is making the affects too narrow so that one way of organizing sliders is superior to another; the effectiveness should be more relative to the players tendencies and the strageic postion of the nation.