Oh, this is my favorite..
I want to be able to set a division into a "No retreat" mode. This could be used to slow down enemy movement, and or slow down the advance of enemy division(s) into a province they have conquered.
Also, division/leader loyalty meter depending on the system of government, fighting morale, and overall individual loyalty and willingness to die for one's country, even in a futile battle that can at the most only slow the enemy down a bit. This meter would support the "No Retreat, No Surrender" order.
So depending on how loyal/die hard the soldiers in the division with no retreat mode are, when they ultimately lose the battle, they should either break down, and be assimilated into the existing partisan activity of the province in question, surrender to the enemy, or fight and die to the last man.
Let's take an example.
I'm defending an area of East Prussia against a massive Soviet invasion. I have 3 divisions in a no retreat mode in Köningsberg, when 5 of the Soviet divisions attack and overrun their positions. The survivors join a resistance in the area, and if the Soviets with their surviving troops attack Elbing alone, and without support from a garrison division to watch their backs, they could get owned if i have set my Elbing division(s) on counterattack mode, and they simultaneously counter attack the enemy while the resistance wreaks havoc on the enemy communications/supplies, and make strikes to their supply convoys and retreating troops.
Ofcourse, this should be either very hard or impossible when attempting it in a already hostile territory with high partisanship against you.
If a counter attack like that was to succeed well enough, you should receive either a small infantry brigade or if partisanship is high, then a paramilitary division.
Which brings me to another thing...
Movable brigades!
HoI developers so far haven't seem to have taken into the account, that some countries, such as Finland that are very small, are not able to produce sufficient amount of divisions for each border province for effectively defend it, thus it happens that Kajaani always gets run over, when this is exactly what did not happen.
So what i suggest is this. Make brigades able to be produced, so they are still able to defend against an enemy who comes with massive tank army through a forest, which was pretty freaking stupid of the Soviets, since their formations got beheaded and cut in pieces in the forest roads. Also, a forest should actually give a huge disadvantage to the tank brigades/divisions fighting within it.
Which brings me to yet another matter. Forests should be reduced to wasteland when it's been fought over a certain amount of time by ground forces with artillery, tanks or both. It would ofcourse grow back in enough time, but when a forest province is reduced to wasteland, it's defensive ability against tanks is also reduced to almost nothing.
And yet another thing: You should be able to get the enemy's navy as a condition of the peace treaty rather than have it disappear into thin air or go to the enemy, which is just stupid.
Historical example: My grandfather handed the keys of our surviving Ilmarinen class Armored coastal defense cruiser named Väinämöinen to the Soviets in accordance to the peace treaty.