I think the mod’s suggestion is great actually. Based on the constraints he proposed, I’d say
First DLC: Byzantines and Ottoman Empire/Turks
(Clash of Superpowers and End of Middle Ages DLC)
Second DLC: Indians and Persians
(The Timeless Civilizations DLC)
Third DLC: Japanese and Ethiopia/Mali
(Civilization at the Edge of Known World DLC)
Fourth DLC: Inca and Aztecs
(Clash of the Worlds DLC)
Honorable mention: I really hope the game can also accomodate Spain, Vietnam, Italians (why not, HRE wasn’t a single state either), maybe Hungary / Poland after that if possible