Here's the full list:
Assyria, Austria, Babylon, Byzantium, Carthage, Celts, Denmark, Ethiopia, Hittites, Huns, Inca, Iroquois, Mali, Maya, Morocco, Ottoman Turks, Polynesia, Portugal, Shoshone, Siam, Sioux, Songhai, Sweden, Venice, Zulu.
Here are my thoughts on these 25 for Civ 6:
Huns - replaced by Scythia, will not be in Civ 6
Polynesia - replaced by Indonesia, will not be in Civ 6
Siam - replaced by Khmer, will not be in Civ 6
Denmark - replaced by Norway, will not be in Civ 6
Byzantium - future DLC or XP2, with 99% chance of returning
Ottoman Turks - will be in R&F, i am 99% certain of this
I think only 1 of these 3 returns, 2 at the most, and NOT all 3
Assyria - Replaced by Sumeria, might be in R&F, future DLC or XP2 but low chance
Babylon - Replaced by Sumeria, might be in R&F, future DLC or XP2 with a mid to high chance
Hittites - Replaced by Sumeria, might be in R&F, future DLC or XP2 but mid to low chance
Carthage - i am thinking 50/50 in R&F, but if not then 90% sure future DLC or XP2
Ethiopia - replaced by Nubia, low chance of being in Civ 6 (i realize a lot of people disagree with this)
Mali - I would expect Mali to make a big return. 50/50 in R&F and about 75% later if not.
Morocco - I am not feeling Morocco at all - I think this is one of those slots that will end up as used by one of the new Civ's we got. I guess technically Kongo, even though they have nothing but their continent in common.
Songhai - I think Mali is back in and Songhai ends up being left out.
Zulu - unlikely, but might be in R&F. I think this is a future DLC or XP2 with high chance given Shaka's "mascot" status.
Inca - in R&F, i am 99% certain of this
Maya - future DLC or XP2, again 99% of this
I think zero or one of these return, and if none of them do, then maybe one alternate NA tribe does instead but not in R&F.
Iroquois - Replaced by Cree for R&F, might be future DLC or XP2
Shoshone - Replaced by Cree for R&F, might be future DLC or XP2
Sioux - Replaced by Cree for R&F, might be future DLC or XP2
I am really unclear about what they will do for Europe, and pretty much leave this category alone. I do expect some of these to come back in one form or another, and honestly with India getting the alternate leader instead of Spain - I think there will be one more European Civ in R&F, but I'm just not sure who yet.
Austria
Celts
Portugal
Sweden
Venice - I don't see Venice (or any single Italian city state) returning - but I am now starting to lean closer to Italy as a Civ than I was a month ago.
We are familiar with the DLC/XP model they used for Civ 5. In general I think they will follow a similar pattern. "Do they end up with post XP1 DLC or perhaps an XP3?", that's of course the big question - but with how R&F is shaping up, I am currently thinking Civ 6 will
not have a significantly longer list of Civs then Civ 5 does. I hope I am wrong about that. Time will tell!