The core idea here is that Japan can have ashigaru (normal) and samurai versions of most units. The way to upgrade units of a certain type (spearman etc.) from their ashigaru to samurai versions is to “grant a privilege” to their samurai class, imposing a permanent economic debuff. So say 10% food gather rate debuff to make all spearmen samurai. Call it “enserf the peasants” or something. 10% wood gather debuff could be “Exclusive logging rights” and so on.
The benefits of the samurai variant could be generally in the area of +1 to both armor types and +1 to the relevant attack type(would probably need to be a bit better on more expensive units like lancers and hand cannoneers). So genuinely a net benefit but not something that should generally threaten to make things very unbalanced. I don’t see a need to make it a “tech” with corresponding times or costs and see it as more similar to vizier points. A boost when you need it, though this comes with a permanent drawback instead of being buffs generated over time.
This would support particular timing pushes but have the downside of the economic debuff lasting the rest of the game. Landmarks could interact with the privileges but I don’t see it as critical.
General roster otherwise:
No regular MAA (samurai only and should be a bit more distinct from regular MAA, I’d suggest 2 handed katana samurai with a charge attack)
Horse archer access (with both ashigaru and samurai variants)