So according to fandom there was a “true heavy cavalry” class in the early development of AOE3 but that concept got merged with the “actual light cavalry” (hussar) and the remnants of that class are lancer, cuirassier and oprichnik. Hakkapelit, Harquebusier and Oromo Warrior were attempts to revive that design but none of them worked.
Considering more and more civs added later are given “a second heavy cavalry”, like the unique heavy cavalry for vanilla civs, such as Iron Flail for Chinese, Mahout for Indians, Lifidi for Hausa, and even more for Lakota etc.
If most civs can have more than one anti-cav heavy infantry option, I think it is okay if they also get another heavy cavalry. It’s like how musketeer is to melee heavy infantry.
Also, since the Hakka rework, there is no regular counterpart to harquebusiers and mounted riflemen.
So instead of creating one or two niche units, I would suggest create this pseudo-class for most civs.
The standard version:
Name: this is actually hard. Cuirassier is the most proper name but that name is used. Carabineer collides with the Ruyter upgrade and Carbine Cavalry. Maybe just call them “Trooper”.
Concept: this unit represents the most common, “less armored” heavy cavalry armed with swords and firearms in the 18th century. It served as the regular cavalry for most European nations in this period when light cavalry such as hussars wasn’t popularized (but hussars could still be the standard age 2 cavalry for gameplay purposes).
Role: short ranged soft counter to infantry; no real bonus against anything, but the short ranged attack avoids the heavy infantry melee bonus, so it’s a soft counter.
Design: basically the original Hakka, but Hakka is a faster and more melee-focus version of it.
Avaialbe in Age 3. Compared to Hussar, they could have similar hp, lower damage but ranged and with a small AOE, and also slower.
Availability: depends on the civ design.
Lancer, cuirassier and oprichnik are its unique variants, focusing more on hard countering infantry, splash damage & more hp, raiding, respectively. Hakkas should be reverted to its original design. Mexican Chinacos can be buffed with a lot of different functions and a pseudo-ranged attack, so they already kinda work like this concept.
Other civs:
Japanese and Hauds: they may not be by conecpt strong cav civs so they might not need one.
Aztecs: JRK vaguely fills this role, but it needs some adjustments.
Incas: Inca unit roster is so weird it might take a whole new thread to discuss it.
All other civs already have more than one heavy cavalry option.