Maybe you can insert "General Staff System" tech that would give the first HQ model.
As for combat events, assault and counterattack should probably have much more chance of occuring than other.
For example, offensive focus can give +6% assault, +1% counterattack, +1% breakthrough.
Defensive focus can give +4% counterattack, +2% assault, +1 delay, +1% tactical withdrawal.
I was also thinking about some trench warfare techs, I don't know would that be a good idea. That would be cheap techs with two lines, one which reduces artillery cost (preliminary bombardment -> artillery barrage -> creeping barrage), and other which improves landfort efficiency (basic -> improved -> advanced trench defence)
Strongpoint should be the tech that allows digging in, decreases landfort build time, increases delay and counterattack chance etc. It could have the following specialties:
Key Position Strongholds - centralized execution
Army-Sized Garrisons - large unit focus
Usage Of Stronghold Reserves - small unit tactics
Strongpoint Mentality - individual courage
Fortified Position Defence Training - training
Hope that at least one idea is good