NA Native American: We do have a lot of information about these groups, largely thanks to the fact that contact was relatively recent and most groups still around actively are trying to preserve their culture. However, lack of details is an issue with some groups--most notably the Inuit, who were supposed to be in V but were replaced due to the lack of a known leader.
Perhaps the biggest advantage this group has that none of the other ones do is point 2); a lack of regional competitors. America is the only other nation on the continent regionally speaking, and even if some people don't agree with the idea, Firaxis has stated multiple times both in BNW and Civ VI that regional diversity is something that they look for when selecting civs. it's not a dealbreaking factor, but it does make NA Native American tribes more desirable in filling up an otherwise empty continent. The other point is of course the aforementioned late contact and abundant information. Native American culture is still relevant in modern America, in current events, in history, and in culture; we know enough about many tribes to create distinct civs with relevant leaders and new playstyles, probably the most important factor when it comes to civ design.
The rotational aspect Firaxis seems to be utilizing is a good way of exploring novel playstyles and cultures without overrepresenting a single region. Broadly speaking you could loosely group 8 regions of culturally-similar tribes--northeastern, southeastern, Northern plains, Southern plains, southwestern, Californian, Pacific northwestern (including Alaska), and Caribbean. I think representing two or three of these groups each game, assuming ~45 civs, is a reasonable way to do it.
SA Native American: Going beyond the Inca/Maya (and even the Muisca), there are many tribes still around today; most might think of the uncontacted tribes deep within the Amazon, but regions like Bolivia and Colombia are also home to indigenous groups that do interact with modern society but are just as distinct as northern American groups.
Similar to the NA Native American civs, there isn't much regional competition in South America. However, the SA tribes are far less prevalent in popular culture and the media, other than the novelty of the uncontacted Amazonian groups. There's also the fact that, unlike North America, there are many modern nations on the continent that also want to be represented, in addition to the as-of-yet-unannounced Muisca who in theory should be close to par with the Inca and Maya (admittedly not in popular culture, though). Finally, there's the fact that even though they aren't the same culturally, the current non-Brazil representatives of the region + the Aztecs are already Latin American natives, so the situation isn't the same as in North America; there are in fact indigenous groups of South America already in Civ, as they have been for years.
Central Asian steppe: If you search the mod workshops for a mod civ based in this region, you'll find plenty, and for good reason; there were many nomadic groups based in this area, which had the unique distinction of being recorded very frequently relative to tribes of other areas because of the heavy use of the Silk Road. Different travelers gave different names to different groups, or sometimes to the same group, so there's a lot of information about this area of the world; the issue is separating informative details from exaggerations and misconceptions. Still, though it would be a mistake to dismiss all of these groups as the same, it is true many of them did broadly share a cultural continuity of sorts in being both nomadic and horse-based.
I don't have an issue with the current steppe groups in the game. The Mongols, by consolidating many of them, in a sense do "represent" the medieval-era of the area fairly well. The Scythians and Huns serve a similar purpose for the ancient/classical era; however, I would like to see a more peaceful variant from the area; central Asia is a big place, and plenty of groups played roles in trade and the spread of all the stuff that goes along with that. The Silk Road ran through the heart of this region, and these groups were in many cases active participants in it one way or another.
European "barbaric" groups: Somewhat similar to the central Asian steppe issue, where we have plenty of names and lots of descriptions, but not necessarily very accurate ones. There's also the complicating issue where some modern countries claim ancestry from these groups, some of which are already represented in civ.
Both 1) and 2) are big issues for these groups; for many of them, the extent of our knowledge from historical texts is that they were barbarians and quite destructive. In recent times we've learned more about them, but I'm not sure if it's enough to construct a fully-fleshed civ that would be distinct from, say, the Civ V Huns. As said above there's also another issue for some groups in that some modern states claim cultural continuity with them (for instance, the Franks -> HRE -> Germany). In any case, Civ has the Celts (or maybe a specific Celtic group this time around), which I think is reasonable enough for a region already crowded with civs.
Middle Eastern tribes: Though it certainly wasn't a perfectly smooth process, the Arabian peninsula does have the "advantage" so to speak of being unified quite early compared to the other regions being discussed; Arabia in most iterations represents a caliphate that encompasses the entire peninsula. A non-Arabic tribe in this area would be a pre-Islamic or Biblical one. We do know about them, but as before, there is a vein of cultural continuity between these tribes and post-Islamic Arabia; they're not the same, but there are many points of similarity.
Between Arabia encompassing the peninsula as a whole most of the time and also incorporating some significant aspects of the various tribal cultures, I'd say the pre-Islamic Arabic tribes aren't a urgent inclusion; with that said, keep in mind that this is strictly a discussion of non-kingdom, non-state entities, so this doesn't refer to Israel or other entities of the peninsula.
East Asian (pen)insular aboriginals: Like the SA Native Americans, the aboriginals of regions like Japan, Korea, and Taiwan are not readily recognizable to a Western audience, and in all frankness, to most audiences outside of the nations themselves. The same goes for SE Asia natives.
Many groups have histories reminiscent of American natives, especially in nations like Taiwan and Vietnam, but without cultural context it would be difficult to create compelling civ designs for them; the issue is similar to that of South America, where there are still nations in the region that have yet to debut in Civ that should take precedence.
Australian aboriginal: Probably the group with the strongest case for inclusion not already in the game (ie: outside of NA Native American, European, and central Asian steppe nomads). The reasoning is similar to that of North America; Australia only has one civ to cover an entire landmass, and the aboriginal groups played a role in Australian history akin to Native Americans in American history; additionally, they are currently around and have a distinct culture separate from Australia.
With that said, I'm not advocating for the inclusion of a specific aboriginal tribe, at least not before a number of other civs yet to debut. I was just noting that out of the unrepresented native groups, they have the strongest case for inclusion in the distant future.
Side note: I didn't discuss Polynesian tribal groups because Polynesia as a whole was in V, and while I'd prefer a specific group, that's already more than most other regions on this list.
African native groups: Like the east Asian groups, there are so many major nations, kingdoms, and other political entities of the region that have yet to be represented that it is far too premature to be considering more obscure, less traditionally-structured groups.
(whereas Greece and Rome, and Paris and Aachen are precisely two tiles apart.)
