I wouldn't know, this would increase the micromanagement load on the player significantly. And when it comes to preparations for attack, the preparations were mostly operational planning, not really anything the guys in the division would have to take part in. A dug-in unit on the defence could plausibly switch instantly to the offence, as long as someone had made an attack plan in advance. A system where premade attack plans would actually become relevant is hardly within the scope of Victoria 2, not even HoI3 has that detail level.
In WWII armies where different than in WWI and before. What was learned from WWI was how to attack without long preparations. So in the Victorian era, at least an offensive or defensive stand would be relevant. This should probably be managed at a higher HQ level. Probably the theater HQ.
What would be nice is if you would build army units as battalions, and then be able to upgrade them to regiments, brigades, divisions and corps (these upgrades would also require tech, so in the beginning everyone would just have battalions in peacetime and regiments when at war).
This upgrading and supplying would expensive in peacetime, but when mobilized it would be fairly cheap, and all units would from mobilization grow one level in size and loose one when demobilizing. So a normal army would in peacetime be a lot of battalions and if a major war came along it would be able to grow greatly with the active units.
This would mean UK, USA and colonial forces would be battalions (regiments when at war), where as major continental European forces would have regiments (brigades when at war) or maybe even brigades (division when at war). The simple way of forcing the construction of greater units in continental Europe would be a unit cap. This cap would be that there could only be one unit from each province.
Then when you build a battalion then the commander is AI generated (historically real generals, but you don't manage them), and if a battalion gets a lot of XP, then you would upgrade that battalion.
This all in all would mean if Germany was 100 provinces, they could have 100 units. And if they really wanted a huge army they could in peacetime have 100 brigades, but would never need it.
Also when you conquer a province, that provinces unit would ofc start as a battalion when you build it, so if it takes a year for each upgrade (if manpower is available IN THAT PROVINCE, which is not likely for some time), then it would be 3 years before it would be able to become a division.
So in the end territorial wars would not suddenly make you able to build a huge army.
Late in techs (WWI-time) you would also be to mobilize 2 levels. This would lead to the largest peacetime unit being the brigade.
non-mobilized/1st level mobilization/2nd level mobilization
Battalion/Regiment/Brigade
Regiment/Brigade/Division
Brigade/Division/Corps
The as in HoI3 there would be HQ units, but these would only be Army and Front (Army group/Theater in HOI3) where you would be able to change the commanders and set AI objectives.
But HQ's would not be built by the player, but spawn when his forces reach a certain size. At the beginning each front would have a front commander. When that front commander has a unit total relative to 6 corps (also at 9, 12, 15 corps and so on, each time a new army HQ would spawm) 2 army HQ will spawn from the Front HQ, though these would have NO fighting strength, and you would keep them away from the front lines, but the still close enough to help the units under it.
1 corps = 3 divisions = 9 brigades = 27 regiments = 81 battalion
battalion = 500 men
regiment = 1500 men
brigade = 4500 men
division = 13500 men
Corps = 40500 men
So if Germany still had 100 provinces and had 2 battalions on the Northern Front (vs. Denmark), 10 battalions on the Southern Front (Alps), 40 regiments on the Western Front (France + Low Countries) and 40 regiments on the Eastern Front (Russia) right before the beginning of WWI (with all the right techs researched). Then they would have no Army HQ's in peacetime. Full army size would be about 126k men.
But in wartime they would have 2 brigades in the North, 10 brigades in the South and 40 divisions in 4 armies on each of the Eastern and Western Fronts. Where the full army size would be about 1134k men in 92 combat units and 12 HQ units.
The offensive/defensive stance would be something set at Army or Front level, so it would not lead to a micromanagement hell, but be realistic. So the stance in some way would be the same as in HoI3 just that it would not only be for AI managed units, but also for play managed ones.
All of this put together would mean you could have fun with fighting wars, but it would not become the center of your focus.
Also Marines would be the only units to be able to make beach invasions. These would be permanent pro. units and only be battalion size (because nothing bigger would fit on one ship). Therefore this would also force countries like the US and UK to have several small units. And landings would be harder, since the larger units would only be possible to transport from one controlled harbour to another.
Also tank units and artillery units would be very expensive for their size and therefore these might only be battalions in peacetime, and no country would afford more than a few of these battalion at all.
But support units would be add-able, so as
battalion = support platoons
regiment = support companies
brigade = support battalions
division = support regiments
corps = support brigades
so spreading out armour and artillery (and other special units) in peacetime would make sense, but not really in a time of war. As it was IRL.
A country like Denmark (if Denmark is 10 provinces) might be 2 regiments and 5 battalions of INF, 2 battalions of CAV and 1 battalion of ART.
Also in all countries one of each type of units may be designated as a "Guard" unit. This unit will then get 20%+ in moral and organisation, and will always fight to the end in defence of the capitol. But will never be further away from the capitol than it can get back and defend it. So if you capital is theatened then it will give you a notification of the unit auto-moving back to the capitol. And they would not be able to be transported on ships.
Also if you have a province that has no designated military unit, and the province is invaded a home guard regiment will spawn to hold back the invaders, but will not be moveable, and you will be able to disband at any time you choose (valuable skilled craftsmen might die fighting, in stead of cheap soldiers). When a home guard regiments spawn you would get a pop-up message asking for you to accept this happening. But this would only happen if no other military unit is in the province.