I think it would be really cool if you added a second tier of doctrines for cold war era tactics. You could maybe gate it behind unlocking modern tank and jet fighter. Thing like, say, bounding overwatch (increased infantry breakthrough), recoinassance by fire (+1 recon?), shoot and scoot (artillery HP and org), carpet bombing (increased CAS attack/targeting?), hull down/defensive armor focus (increased tank org and hard attack, less soft attack and speed?) for a tree under superior firepower to represent NATO doctrines.

Not sure what tactics are applicable to Soviet cold war tactics, most I can find on the internet discusses strategic scale ideas, but it seems they would try to rapidly overrun Europe by creating breakthroughs and exploiting them to target supply etc, as a continuation of the deep battle idea. I guess a mobile warfare style tree emphasizing especially mechanized infantry, maybe rocket artillery, cheaper armor and offensive armor?, and special forces would be the way to go.

A guerrila style tree which wouldn't be great for players but AI might often pick if they aren't a strong country by the time they hit it, where the focus is increasing resistance, slowly giving in ground, surviving encirclement, maybe resistance to CAS, and possibly a few tactics like human shield, child soldiers, IED, and increased dig in?

Not sure what would fit a good fourth tree, maybe something focusing on Chinese military doctrine, or possibly a strong defensive tree emphasizing high tech and low manpower for countries like Sweden, South Korea, Japan (not really limited manpower but maximize power projection within article 9 limitations)?

Naval doctrines could split between focus on saturation attack with missiles, stealth technology, and emphasis on smaller ships for say Russia/China, U.S. will focus on big as ♥♥♥♥ carriers, CIWS (Maybe increased ship evasion or HP to represent it?), and fleet range. Not sure what sorts of modifiers in game would be great for countries with small navies mostly acting as coast guard and little/no blue water navy.

I wouldn't know where to start with air doctrines, honestly I assume most countries had the same goals in mind.