Day of suggestions, today?
Here, mine, is the aim that in all game all strategies should be viable (rush, turtle, boom).
In phase/tier 1, I thought about a RPS system: -Spearman could be the only unit available until phase 1.5, that in numbers are enough tankier to take CC and towers.
-Cavalry skirmishers are good against them, but lose to structures.
So spear infantry>towers/cc>skirmisher cav>spear infantry.
Blacksmith allows tier 1.5 could be a strategic decision buildable in phase 1: to build them or make another barrack.
-Unlocks infantry skirmishers, with good dps against infantry in general and against ranged cav. Shorter range than towers so they are good for raiding and defend, but not against defensive buildings.
-Unlocks also cavalry swordsman, good against ranged infantry, slightly better than spear infantry against buildings. Could be soft-countered with skirmisher cavalry with hit&tactics.
-Allows military techs in phase II
*Swordman, slingers, and the other cav available in phase II or in tier 1.5?
Romans and Iberians could be subvert this: Romans could train swordsman (Good against infantry and also better against buildings. Countered by well guarded or hit&run ranged infantry, and ranged infantry.) and Iberians skirmishers instead of spearman in phase I.
You then you can try to rush with spears/cav skirmishers, turtle up with towers. At mid phase I you get 2 units that counter each other, being skirmisher a good defense unit but also support unit to rush. Or you can skip and go to phase II for better eco tech and better soldiers.
The problem by design is that booming allows a good mass of soldiers that can defence. Making skirmishers less common in phase I would solve this without dropping c/s concept. Also barracks soldiers could start in advanced rank so choices between training them in CC or barracks have actual consequences. And also targeting them when raiding. CC could train only women and spear infantry and one limited explorer unit (cav skirmisher skin without javelins). Spearman trained there could go with milita status (can't rank and military upgrades doesn't affect them)
Let's hope that some ideas help someone (either the base game or some mod)