1.I would like to be able to buy slots with prestige points. In this way you could choose to have larger armies or smaller ones with better units.

2.For the german campaing I like the option of chosing different special forces for example:
Heer-> Big Artillery guns , Sturmpioniere, Großdeutschland division
Luftwaffe-> more speciallized aircraft (maybe transport aircraft for fuel transport, more transportaircraft), Fallschirmjäger
WaffenSS-> Heavy tanks or only 15 points health for heavy tanks, SS motorized infantry, volunteer infantry
So you can get maybe 10% different units and a different playstyle for each campaign.

3.Combined arms bonus. Units should give each other bonuses when they have a high enough expierence Level or are trained for it from the start. Example: You have a Wehrmacht infantry unit that is trained in combined arms so having a panzer unit near by gives a bonus tto both the panzer and the infantry when they attack. A normal Wehrmacht infantry unit doesn´t get this bonus if it doesn´t have the training for example a volkssturm unit. This could be interesting in the Barbarossa campaing where soviet tanks are better but with this bonuses the axis player can simulate the higher training (combined arms) and overcome them. As the war progresses the soviets gets these units too and the german get some units without these trianing bonuses. These could be aplied for arty and air to, for example an expierienced wehrmacht infantry unit increses the damage from arty and aircraft when they attack an enemy unit nearby thus simulating that they are trained to correct the artillery fire and the close air support.

4. Artillery should be able to give fire support for entrenched unit, thsu simulating a stactic defense where the defender had time to coordiante with the backward artilery. Maybe the artilery has to be entrenched to. A nice bonus would be to give entrechned unit a oversupply order thus having more ammo.

5. I like the distinction in heavy arty you annouced. I always wondered about the littel difference in damage between 150mm and 105mm shells in Panzercorps1. Maybe light arty can give Speerfeuer like in Panzergeneral2, would be cool if heavy arty could give counterbattery fire ( albeit with a malus because the arty is not fully visible e.g. you fire at a target you hav enot spotted, when you have a unit near the target you could count it as spotted and thus the attack will do normal damage)