Couple things I would like to see.
1) Brigades. These were historically, the smallest unit that stayed the same. It may make it more complicated, but the other thing I would like to see would help.
2) More interactive Brigade/Division/Corps/Army management. Make it so that you can, if you wish, change who is leading each brigade, division and corps in your army group, so that more of your generals can gain experience (Albeit slower if they are not the general commanding). With Brigades, it would require a Brigadier rank also. For there never was just one guy at the top, you had all those commanders, all the way down.
I would like to be able to customize my divisions basically, have the 1st Armoured Brigade have 2 armoured brigades, 1 motorized brigade, 1 artillery regiment or so on, for example.
And the ability to have independent Brigades, so you don't have to send a whole division off to garrison Greenland, yet have a mobile unit, unlike the current Garrison.
Of course, it could all be set to be automated for those who do not want that level.
3) The tech tree could be expanded, slightly, but I found the HoI Tech tedious, so I like how they grouped them together in HoI II, so you research the techs automatically.