As a Civ player since Civ2 I have adjusted to the anachronisms of Teddy Roosevelt warring with Cleopatra and Victoria fighting the Maori. Old World is much more immersive and consequently more enjoyable because it presents a truer historical and geographic setting. I suppose one could add a Germanic or middle European tribe, possibly a near eastern empire but I really would not stray far from the time and place of the current game. More acceptable would be a complete set of alternate empires such as the Native American tribes (either North, South or Both), maybe an Asian pack or a Northern European set but would really oppose adding a single Civ outside the time and place of the current set.
I agree that grounding the game to it's historical-geographic context is fundamental, but the "map" could be extended to cover all the eastern world at the time as well. In fact, India and China were not that much detached from Rome or Persia, for instance. The history of ancient governments/nations/empires actually covers almost continuously all the temperate/semi-desertic/semi-tropical areas of eurasia and north africa. I would have no problem in seeing something to cover for different parts of India or China. The mesoamerican-andean idea is interesting, but as a fully separate game setup (I would not like to marry my assyrian daughter to a mayan landlord...). Tribes also add a whole new nice level of content.
The civilization series injured a bit the approach to the political entities a player can use. And the new humankind does not seem to tackle this issue very weel (they only have the marketing-flavoured "1.000.000 civs to play with!" which, honestly, seems awful). All of these games have a "blobish" historical concept acting as a player's vehicle to the game. Old World has the same BUT it makes sense (historically speaking) and the families give it a whole new level. That being said, i don't think that the game would benefit much from extending the number of different nations up to infinity as Civ series did. It was ridiculous to see the Huns setteling cities and behaving "like romans" on civ V (the same follows for scythia on CivVI or for "native americans" on CivIV). To see such phenomena in Old Wolrd would be distructive IMHO. Given that this game has a great historical immersion, I guess that the "new nations" have to cover not only the names of historically recorded populations but also the different socio-political and techno-economic realities that covered eurasia c.2500-1500 years ago. This would include the "empires" that we see already but also tribal confederations and (*personal wish*) nomad pastoralists. This does not differ much from what CK2 did with the feudal-tribal-nomad thing. But in Old World such distinction would reflect a whole different way of interacting with the hexagonal tiles and its yields.
To sum up, adding "original" new content, with new ground economic mechanics that serve as the backbone for the new "population names" that usually take part in this game, would be the best thing that happened to this type of games. Adding simple "population names" with just a different combination of families would be awful.
Anyway, some new nations can still be added to the game without extending the historical/geographic background, but I think that all of them have some few problems:
Hittite is a classic in a certain way, and it could cover the empty space that the Premade Map has on Anatolia. Nevertheless, I am not sure that the historical record is sufficient to insert the family concept on this ancient political enity.
Armenia has had some interest from some civfanatics (and even a few mods). Still, the proeminence of the armenian government is not that much dilated in time and space; I don't think it would be the best option.
Aksum (or
Ethiopia if you prefer) could cover the "real" extension of the ancient western world, but I think it can suffer from the same problem as the Hittites.
Kush, but maybe it suffers from the same problem as Armenia.
Extending the geographic range of the game, but still keeping the same concept of Nation, it would not be bad to see:
Maurya, reflecting the political entitie(s) of the Ganges basin of northern india. It could even (maybe) use the traditional vedic castes as families.
Tamilakam to reflect the territory of the Tamil Kingdoms in sowthern India (although it may be dificult to find family names without incurring into medieval history?)
China (I know blobs are not so cool, but china had to some extent a recurrent political unification under different dynasties; such dynasties could be integreted in the family system)
