I would rather they introduced new military units that can also heal units and carry Relics for other regions instead of just giving Warrior Priest regional skins. For example, we in the scenario editor can introduce Sohei for the Japanese, a warrior monk who uses a naginata and specially has an attack bonus against cavalry. Also, Hospitaller can be introduced in the scenario editor as a mounted warrior monk, similar to the Missionary but can fight and cannot convert; Shaolin Monk as a warrior monk with the shock infantry armor class, wielding a staff and running fast.
Although the Aztecs have Warrior Priests in AoE3, that seems to a reflection of the bloody ritualistic methods rather than meaning that the Aztec and Maya priests were actually warriors. Although the Rus (Slavs) have Warrior Monks in AoE4, that seems to be a reflection of Alexander Peresvet’s personal deeds, not that the Orthodox in Rus did have some kind of warrior culture.
It’s not that complicated. Having a unique identity and playstyle for a civ doesn’t need to have every building that different from every other neighbor in the region. By the way, the Aztecs don’t have Ring Archer Armor so technically their Elite Skirmishers can’t be fully upgraded.
I’m not sure it was an appropriate decision to share Chasqui with Chimu and Muisca, because as far as I know Chasqui is exclusive to the Inca Empire.
In my concept, I would make the Slinger a regional trash unit for not only Andean civs but all American civs after adjusting the stats, and I would like to rename and reskin the Eagle line units to Scout Infantry, Light Infantry and Shock Infantry. Then, the Aztecs could have a new Eagle Warrior as a unique upgrade to Scout Infantry, and the Incas could have Waraka Slinger as a unique upgrade to Slinger in Imeprial Age and could have Chasqui as a unique scout unit that is trainable in Dark Age and can be automatically upgraded to Scout Infantry when hitting Feudal Age.
Purepecha can share the architecture set with the Aztecs and Mayans, and get the faster moving Scout Infantry and turn their gold cost into food through UT while there is no Shock Infantry upgrade, so the fast trash Light Infantry can be their identity.
Chimu can share a new Andean architecture set with the Incas, and have the Maceman in Blacksmiths and the Bolas Warrior in Castles as their UUs. With a UT, their infantry can cost less gold, so their fully upgraded Shock Infantry can be featured at a cheaper gold price.
Muisca can get a new wooden architecture set or at least have the Andean architecture, and have extra sources of gold and the fast castle/imperial strategies that develops from them while there is no Arbalester upgrade. Make their Scout Infantry line cheap and fast to upgrade or even free to upgrade, so that it can have a distinct gameplay when combined with fast aging up.
By the way, for a Bantu civ that will also have access to Scout Infanty line, their UT Assegai can give Scout Infanty and Skirmisher units +50% bonus attack, and allow Scout Infantry units to have a charge bar to throw their spears a range.