Haven't actively contributed to the mod for a while now, but I've been thinking about the shift from regiment to division as the decisive unit from 1790 to 1860-ish, and I may have a solution. It'd require a revision of the way land doctrines work, and for that matter the way land brigades are used, more like HoI3 than HoI2.
Create a command-and-control segment in land doctrine techs. In 1790, a "division" consists of a single brigade, consisting of a single regiment (in the case of British regiments, for instance, this would really be a single battalion, but we'll operate under the simplifying assumption that the regiment has two battalions, a rear-depot battalion for training replacements, and a deployed battalion that the game reflects as a unit).
Revolutionary and Napoleonic "brigades" should be fairly weak, to reflect the fact that they're not truly brigades. Sometime between 1790 and 1815, "brigades" get a stat bump, to reflect the first use of mass maneuver troops in the French Revolution's foreign outcomes rather than fighting more-or-less exclusively by regiment.
Between 1815 and 1865, a tech should be available in the C2 category for a binary division of two brigades, and between 1865 and 1914, a triangular division, which should be the default. During the Great War period, square divisions (the HoI division-plus-brigade division) open up, and late or post-WW2, pentomic divisions (one of the silliest ideas ever, really) open up. In all of these cases, "brigades" are broken up as follows, more or less:
Infantry family
- Light infantry
- Infantry
- Motorized infantry
- Mechanized infantry
- Infantry support tank (e.g., PzKpfW IVA through F)
Mobile Family
- Cavalry
- Light armor
- Armor
- Heavy armor
- Air cavalry
Enhancement Family
- Anti-Aircraft
- Artillery
- Self-Propelled Artillery
- Engineer
- Airborne Engineer
- Assault Engineer (covers Commandos, Raiders, Rangers, Sturmpionier battalions, etc., on a conventional battlefield)
- Military Police
Special Role Family
- Mountain
- Marine
- Airborne
One problem with this, of course, is that certain divisions are going to be typecast, only being able to take brigades of certain types - mountain, marine, and airborne come to mind, especially airborne, which has a designated role that no other unit can undertake in HoI2-engine games.
The other problem is the fact that HoI doesn't like disassembling and reassembling brigades when you've got more than one. I don't recall ever detaching, say, an AA battery from a battleship that's also mounting a radar unit.
More of this to follow, I'm sure, this is kind of a notepad post.