Brain-bustingly stupid this suggestion may be, the answer MIGHT potentially lie with the fellows who made Rise of Nations—Big Huge Games. We can look at their current “flagship” title, Dominations, where the German unique unit for AoE3’s time period is called the Junker. Using an entire social class for a single unit doesn’t fit THIS game, though, but the core of an idea could be there.
Wikipedia tells me that the title of “Fahnenjunker” was reportedly the name given to junkers who joined the army as junior officers instead of inheriting estates or joining the civil service. It still exists today as a rank in the German and Swedish armed forces. Junker itself remains a military rank in Georgia.
That use of a whole military rank as a basis for naming a unit, in this case, an expensive musket infantry (can this be a 2-population unit like the Soldado? Perhaps it trains a bit faster and moves a bit faster since it’s Prussian, but can’t toss bombs), has precedent in this game, albeit only recently. We were given the Poruchik, which is the Russian term for a Lieutenant—who apparently commanded Streltsy historically? Huh. Yeah. Why not? Make the Fahnenjunker a unit like the Sentinel or Soldado, with some funky Teutonic quirk that distinguishes it from those two.
We can keep the Landwehr as it, I like it and think that it has a niche. What they add, I’ve come to realize, is not some weird ersatz musket usage, but something else. Higher base damage and flatter multipliers, plus a slightly cheaper overall cost, mean that Landwehr trade better into artillery, hand cavalry, and other skirmishers than Needle Gunners do, particularly in larger masses. It’s akin to the Maltese Arbalester, but less pronounced and specialized. Musketeers are better against cavalry yes, but Landwehrs should be supporting other units against Hussars and their ilk ideally—and Landwehr still have the advantage over Musketeers when facing down skirmishers. Infantry masses shouldn’t be facing down cannons alone, yes, but enough footmen and you can wear the cannons down eventually, especially in Fortress Age.
Japan can keep their Yumi Archer. Kyūdo is still an important cultural and athletic tradition over there. They can keep the Yabusame (once again naming a unit after a whole social thing instead of just its function, but the line blurs here) as well, since that’s also persisted as a cultural/athletic pursuit to the point of showing visiting heads of state Yabusame demonstrations. Giving them something like the Kihei from Wars of Liberty via a card or just straight up in the Barracks in Age III is okay. I guess we can lock Kiheitai behind an Age-Up to make India slightly more unique—now they’d be the only Asian civ with a Commerce Age Skirmisher. As for maybe giving them a gunpowder cavalry, what is the trade-off there? You would inevitably have to lessen some of what
makes the Yabusame a hyper-mobile Culverin in practice. I guess you could be a troll and name this hypothetical unit JUST LIKE the Yabusame, and call it “Hōjutsu”.
As for China, I am in fact advocating for a wholesale unit replacement. The combination of range upgrades, Repelling Volley (though I’d also give that card to a hypothetical Korean civ), and Old Han Reforms add enough pizzaz to the Qiang Pikeman where I don’t really wanna mess up anything with the Old Han Army. The Changdao Swordsman is significantly less unique or special, it’s just a worse Halberdier. Turn that emmeffer into a Revolutionary clone or something similar with a properly Chinese name.
Palace Intrigue → 
New Order Infantry → Nizam Fusilier (retrainable units, replaces cards)
Rangers → Ranger (full replacement)
