I for one like the fact that South America is receiving more than 2 Civs. The fact that the Mapuche resisted two "global" powers such as Spain and Inca and cover a geographic region not yet explored is a great inclusion in my opinion.
I don't think the Mapuche 'resisted' the Inca. There's a report that there was a battle between them but given how little documentary evidence there is for the pre-Colombian period it's not clear the Inca ever had territorial designs on areas south of the Maule - the Mapuche just mostly fell outside the Inca sphere of influence.
I think a lot of the reason the Mapuche weren't a popular addition is that they were in the series before the Inca and are perceived to have taken a slot from the Maya, as Firaxis has traditionally treated South and Central America as the same geographic region in terms of civ representation. Personally I'm not a fan of civs representing ethnic groups rather than kingdoms or empires, but in an area like South America that's largely going to be unavoidable.
I agree with the rest though I think it will come down to either Assyria or Babylon with maybe Assyria having the upper hand to return but Babylon has the legacy factor too being in Civ from the beginning.
I value Civ tradition and think Babylon ought to be in - they keep bringing back the Zulu for no other reason than they were in Civ I (though Shaka is a popular figure as a Civ leader in a way that none of the Babylonian leaders have been). Babylon certainly belongs in the series, but I'd hope that if they find a way to add more civs to Civ VI (which I don't particularly expect at this stage) they treat them as a culture civ to both better-reflect their legacy (such things as the way we measure time, early legal codes, and wonders like the Ishtar Gate are cultural more than scientific achievements) and distinguish them from the Sumerians.
The only Civ that leaves me disappointed it was not included in Civ 6 is... the Celts!
The Celts were a very divisive civ among the community, as they didn't represent either a real state or a (once)-living cultural group but are a material culture defined by archaeological remains. The Civ V implementation was especially unpopular, as a mix-and-match civ of assorted island 'Celts' with mostly non-Celtic cities.
The modern notion of 'Celts' as a cultural identity linking Ireland, Scotland, Wales and sometimes a couple of mainland cultures is an anachronism - there was no such historical entity and the modern cultures of these societies have different and mostly more recent origins.
Is not Scotland supposed to take Celts place this time?
Geographically, but the Scottish civ isn't Celtic (and, obviously, also doesn't incorporate Ireland). Scotland in game represents a culture dominated by the modern Scots, whose cultural ancestry is Germanic.
Given that the only real reason to have Celts in the game is as a sop to Scottish and Irish audiences, I think it's safe to say the Scots have replaced them.
Quite possibly, but the same type of thinking could lead to Aztecs taking the place of all other cultures and civs in Mesoamerica or the Scythians taking the place of every other pastoral group in western Asia or Sumer taking the place of every other culture or civ in ancient/Bronze Age Mesopotamia.
It's more a case of Firaxis reacting to community dissatisfaction with "dustbin" or "blob civs", of which the Celts were among the last to be excised. They'd previously replaced the Vikings with named Viking-era states and the "Native Americans" with named North American indigenous groups. Most civs now in the game represent a single defined territory, state or historical culture, and replacing "the Celts" with Scotland follows that trend.
Oddly, the two exceptions have both been introduced in Civ VI: the Scythians, like the Celts, are a material culture rather than a definable group of people; and more strangely still they actually took the defined Carthaginian civ and turned it into a Phoenician blob.
