I hope the Mapuche don't make the Inca less likely.....they aren't very similar except for being from the Andes region. The Mapuche weren't empire builders or constructors of a vast road system like the Inca. They are portrayed as fierce resistors of colonialism.
Sumer could've replaced Assyria from Civ5. There's still hope that we could get two ancient Mesopotamian Civs. And let's face it, Civ 6's Sumer is the Epic of Gilgamesh Civ (aka not based on the real history of the Sumerian people). I would a more historically based Mesopotamian Civ.
Ethiopia and Nubia share little in common except for being "East African". If it was Christian Nubia (Nobatia/Makuria/Alodia) being represented by an official Civ, maybe Ethiopia would be considered too similar for inclusion.
I agree with all of this, but I doubt that it will be of any importance to the choice of Civs to include in the game. ALL civilizations that last for more than a generation are far more diverse than the game allows: that's the price of Locking In each Civ to a single set of Uniques and a single (or pair of) Unique Leaders.
Shuckee Gee, I studied history at university with no particular emphasis on the ancient Near East or Mediterranean, but as potential distinct civilizations other than the semi-mythological Sumer we got, I could suggest Babylonia, Neo-Babylonia, Assyria, the Hittites, Mitanni, and later in the same general area the Lydians, Medes, Palmyrians, Phoenician city-states, or early Armenians.
But let's face it, the selections so far have been based on distinctly non-historical factors:
1. Do they have a recognizable and distinctive Leader
2. Are they either well-known to the modern gamers who are potentially going to buy the game (England, USA, China, Korea, Australia, Brazil, France, Russia, etc.) or are they so historically important you can't legitimately leave them out (Greece, Rome, China, Britain, France, Egypt), or do they show off the Designers' knowledge of obscure 'Civilizations" (Mapuche, Scythians, Nubians, Cree) to impress the Public.
3. Are they sort of a little bit geographically distributed around the world, but especially representing every place where people live now who might buy the game - so Africa is going to get Short Shrift regardless of the historical importance of any Civ south of Egypt.
And finally, since they can pick and choose what aspects of any Civ to display in the limited number of Uniques, the actual historical aspects of the Civ in question over time are largely unimportant: Ethiopia, for instance, could be the Italian-bashing anti-colonials of the late 19th - 20th century or the Christian Nation of the early middle ages, France can be any France from that of Catherine (who was actually Italian) to Louis XIV to Napoleon (who grew up speaking an Italian dialect) to Charles De Gaulle - and so on and on through just about every civ now or potentially in the game.
Don't get me wrong: I like the 'obscure' civ choices, and I don't even mind the choice to 'showcase' obscure eras or traits of 'popular' Civs - both sets of choices have a gameplay and historically educational function (Admit it, how many of you could have named ANY Scythian leader before playing the game?).
But it means the choice of Civs to include doesn't rest on their 'uniqueness', because that can be manipulated, or their Importance, because that is relative, and everything is subordinate to marketing decisions (Nazi Germany was extremely important in 20th century history, but don't hold your breath waiting for Adolf as an alternative leader for Germany! - and we all know the fate of any potential Tibetan Civ has nothing at all to do with any historical or gameplay relevance)