This is my proposal for the implementation of diverse skin pigmentation for the civilizations of Age of Empires II: Definitive Edition. Although making whole new models for the units of different civilizations might be a bit of a tall order for the development team, I think changing the skin and hair colors of the models already in the game would be enough to convey ethnic diversity. The pigmentation sets I propose are as follows:
Blond: For Northern European civilizations (e.g. Vikings, Lithuanians, Celts, and Teutons). Admittedly the least necessary of the sets, but I wanted to see my own hair color represented in the game.
Brunette: The one already in the game. Would work for Central and Eastern Europeans (e.g. Britons, Slavs, Maygars, Franks, and Bulgarians).
Olive: For Southern Europeans (Italians, Spanish, Byzantines, Portuguese, Sicilians, and Turks) and East and Central Asians (Chinese, Japanese, Koreans, Mongols, Huns, Cumans, and Tatars).
Bronze: For West Asians (Saracens and Persians), Southeast Asians (Khmer, Vietnamese, Burmese, and Malay), and Native Americans (Aztec, Maya, and Inca).
Mahogany: For Northern Africans (Ethiopians and Berbers*) and North Indians (Hindustanis, Bengalis, and Gurjaras).
Ebony: For Africans living closer to the equator (e.g. Malians) and South Indians (e.g. Dravidians).
(*) Berber-speakers actually come in a broad range of skin tones, from olive to ebony, but I chose mahogany as a sort of median or average for them. A better alternative, if possible, would be to have them randomize skin tones so that some would be lighter and others would be darker to better reflect their diversity.