Japan Civ, Worldly, Spiritual, bellicose.
Unique building, embassies, wooden fortress.
Historically and today, Japan is crazy good at learning from people. Crazy good. Embassies grants them bonus that they can choose in every age. Tang, Dutch, Portugal, Inward perfection. That gives out various bonus to eco.
Wooden fortress/castle/keep, quick construction of forts, not like huge stone building but using mix of wood and stone. Cheaper but also have less health. Or straight up stone. nerfing forts might be too much. Fortress could also boost eco or units since they are cheaper and weaker at health.
Age up buildings, base on various historical buildings built by emperors, shoguns, and religious things. Tech boost, cheaper monk, cheaper unique units, cheaper castles, castles also act as multi-purpose econ buildings.
Age 1/2: Upgrades, Tenno or Bakufu structure, as historically, the power struggle between admin bureaucratic aristocracy verses Samurai warrior class. Mill boost or cultural/tech boost. Produce tenno or shogun. Eco/priest unit verses mill boost. Since tenno had no power historically, make it a vill who has to gather his/her own food. Ending of the Tenno power, rising of Samurai shogun. Later Tenno can be like a cultural figure head or something.
Scot = Ninja, extra damage to villagers and buildings, but on foot thus slower than actual scots and give the vills a chance to run away. Maybe can build bombs to blow up walls in age 4. Can find extra upgrade in black smith or barracks, or town centers.
Early samurai, since early Japanese warfare involved heavy armor Nodachi wielding samurai, give them a man-at-arm that does high damage but low health early game. And can produce more regular samurai later.
Basic spear, archer, and light cav do exist of course.
The idea is that if there’s economy/gimmick to produce too many units, then maybe keeping what you have for longer could also workout. Give them extra speed with shogun unit.
Maybe early monk age 2? But could not do attacks?
Age 1 enables Embassy.
Age 3: Christian buildings, Buddhist buildings, something buildings. Ramping up to Sengoku jidai and through it.
Unique units base on various choices. Embassy or age up building.
Japanese warrior monk spear/ or heavy infantry. Boots attack and/or defense. Infantry speed. But cheaper than regular priest. Cost other resources other then gold.
European priests; healer and fast. Regular priest. Lowers unit promotion cost.
Waco pirates, a unit producing ship that’s good at burning buildings.
Versitility of Samurai units. Since historically samurai as a class of people and used variety of weapons, we can either give the units a promotion system where if they killed enough then they get promoted to samurai units with additional health or extra type of damage, like samurai archer gets a melee sword, so is gunpowder units, and maybe better health.
Samurai men-at-arms, high damage, but low health compare to regular man-at-amrs due to lack of shields. Single targeting, fast moving. Low heatlh, cheaper? Like Chinese palace guards. But can get promoted rank.
and heavy cav (not samurai cav yet) Just regular cav, can become promoted unit.
Nodachi samurai still exits, but also allow the production of normal samurai.
Unable promotion system where units gets to become a “promoted” unit as in Samurai spear, or Samurai archer, or Samurai Cav.
Since heidiyoshi banned social mobilization as in class switch, the castle/imperial upgrade could allow you to produce samurai units and not just through upgrades.
Of course, mangonels, trebs, and ballista
Age 4: Structure that represents inwards verses outward learning/expansion
Portuguese tercios, a specialize gunpowder units that is more expensive, but maybe fast, or more health or gets armor boost when near spearmen?
No more promotion units but can outright produce promoted units at a cost base on age up choice. (base on age up.)
Bombards, and also a cheaper culverin that requires slower assembly time that fires fire arrows. Maybe a heavy gunpowder unit that fires big shotguns and it can be a Japanese ribauldequin
Historically, Japanese economy went from barter system in Heian period to promoting free trade with a actual trading system in Sengoku. So maybe embassy gives out trade bonus also.