The best way to continue with a division construction feature in HoI4 would be to break every unit type into one size- battalions (650-1000 men). Land Doctrine would prescribe a propensity for binary, triangular or square divisions. Which means 6, 9, or 12 maneuver battalions, respectively. Then a truly fully capable division would require the standard support units: *Arty, AT, Eng, recon, etc. (*There should be room for light and heavy artillery battalions, generally a mix of 3 or 4). Additional room could be given for an extra battalion of your choice for increased ability in one area, such as an extra tank or engineer battalion.
While it sounds like unnecessary micromanagement and bland one size-fits-all divisions (which we tend towards anyway), it would mean a big step toward realism. It would also take a lot of the guessing game away from the construction feature.
One thing than needs to be present for the unit builds to be less random and more realistic is a split "strength" into "men" and " heavy equipment". Retreating units should conserve their men but lose some equipment. Divisions low on fuel (i.e. breaking out of encirclement) and supplies may lose all their equipment very fast. Things like the "Panzer army" without Panzers would become possible.