Since this Vicky goes into the interwar years i really hope that the way you fight wars change greatly with techs. This so as to smooth out the huge developments in how wars where fought in post-Napoleonic times and up to the First World War.
Doctrine should make a huge difference and you should have to change your strategy compared to new techs. One year Blitzing forward would be in and the next digging in for attrition warfare would be the thing to do. Also when one country develops a new thing (like HMS Dreadnought, the machinegun, rifled weapons, tanks and aircraft) other countries should be able to go forward in these fields a lot faster.
All units should only be managed at divisional level and in a relative simple way. But you should be able to give your divisions orders of how to attack and defend. Something like offensive/defensive stands. If a division is set to "defensive" and is attacked it should be a lot better defending than a unit set to "offensive", but when attacking the "offensive" units should be alot more effective. It was a problem in Vicky1 that you could dig-in to insane levels and then when a oppotuinity presented itself you could suddenly attack without long preporations like it was IRL. Also if you have maybe 5 settings for each division then it should take some time to move from one setting to another, maybe a week. Then if you are going from defensive to offensive, you will have to pass semi-defensive, balanced and semi-offensive, and thous it would take a month before you could make a massive offensive, but with the enemy hurt after attacking you, you could still make a limited counter offensive.
This would make the western front in WWI make sence, but since the eastern front was a lot more mobile you would as Germany never set your units there to "defensive".
These stands could have influence mainly on organisation and movement speed of the units. "Offensive" units being faster moving and have a small org. bonus when attacking and "defensive" units deing slower, but having a org. bonus when defending, and also digging in faster and better, and also keeping morale higher when retreating.
And then i just hope this will be sinked in a good way with HOI3 and overall military history, because 1864-71, 1905, 1914-18 all created drastic changes in military thinking and therefore there shouldn't be baby steps ind Vicky 2 and then a huge leep forward come 1936. I would actually be great if some of the HOI3 techs would become available in Vicky 2 or some other way of linking the overlap if fx you won WWI with Germany.