The Nubians were known as far back as the Ancient Egyptians Old Kingdom era (in fact the Nubians adopted Egyptian culture and customs, and worshipped the same gods as them until Christianity showed up), and they survived well into the medieval era, repulsing Muslim armies sent against them from Muslim Egypt (which was conquered from Byzantine control, and thus severing the Orthodox Christian link that the Nubians enjoyed having with the Patriarch of Constantinople).
This is best illustrated that the Age II hero, Archer of the Eyes in the Saladin campaign, represents Nubian archers that Saladin and his Ayubbid successors hired as mercenaries to their armies due to the famed skill of Nubia’s archers.
Athabaskan peoples of Alaska, the Mississippian peoples of North America, the Eskimos, the Anasazi and “Pueblo” peoples of Southwest United States, … and many others I have looked at at studied. I am not a Native American scholar and do not pretend to be. But I do dabble here and there.
I have also taken a look at several of the Polnesian and Central African civilizations (like the Bantu people) and South African (like the Zulu people) that other persons in this forum have proposed as new civs.
I am certainly not against adding more Native American civilizations or African civs into Age II as long as they have adequate background and would be excellent new additions into the game. Key word: excellent i.e. they need to have
→ a proper Wonder that makes sense for their culture,
→ their military units they produce must make sense for their respective civilization’s abilities (Mayans, Aztecs, and Incans pretty much succeed in that regard because for one, they actually had properly trained militaries that gave the invading European conquerors a run for their money and they were also reasonably well recorded by their own illustrators and artists but also by the Europeans that conquered them)
→ their architecture must also be grounded in reality. (I actually have an issue with Huns sharing Central European architecture as I think they should get the “ruined architecture” that exists in campaign scenarios. Goths are fine as them sharing the Central European architecture alludes to the fact that their descendants became Germans, Italians, Spanish, and Portuguese)
→ the new civs must have something “imperialistic” about them
→ and finally, these new civs history needs to be compelling and fun enough to have an actual campaign made about them.
Aztecs, Incans, and Mayans have well-recorded histories and stories compared to the rest of their Native American brethren, have built buildings that are “Wonder worthy”, have sophisticated militaries and economies, and also were “imperialistic” in the sense they created empires and found other nations and political-warrior entities.
Frankly, so many of the peoples proposed to be “new civs” are too isolated in their cultures, too far away geographically to have been part of the main events that characterized the “medieval world” and/or remained relatively underdeveloped Stone Age societies that merely got by with plain hunting and gathering, fishing and agriculture.
The Mississippian people is by far the closest civilization to fit the bill as a “new Age II civ” as research indicates that they did have something close to sophisticated cities and political organization (where the “imperial” element might come in"). But sadly, we still known so little about the Mississippian civilization as compared to the Aztecs/Mayans to their south.