SMcM
Emperor
I’m going to make a guess, but I think that the last pack is going to be a returning civ, and there are only 2 places I think it could be at: Portugal and Native America. Out of these I think it could be Portugal, The Iroquois, and the Shoshone.
Shoshone I would rule out tbh, it's not an especially obvious choice when compared to other native American groups. If people are under the impression it might become a series regular that is in my opinion a perspective distorted by it being in the most recent Civ game before this. If anything, I think it being in Civ 5 makes it less likely to show up here, since it's been featured recently and it's more likely they will pick a different group since Shoshone is hardly so significant that it's irreplaceable. For the same reason I'm not thinking Cree is unlikely to reappear in the next game.
I think speculating first featured in Civ 5 civs might return just because they were in that game doesn't make sense really. Yes some of them have returned, for instance Brazil being in the base game, Indonesia and Poland in DLC, and Sweden in an expansion. And those ones seem to be potential regulars for various reasons- partly for marketing reasons I'd say- aiming to represent countries with large populations or significant video game popularity, and partly just trying not to leave significant parts of the world unrepresented. It seems some representation for Polynesia will be in civ games now (Polynesia and now Maori), and following the lead of Mapuche and Kongo I think we can expect more variety in South American and Sub-Saharan African civilizations in the future. But, Assyria seems to have essentially been swapped out for Sumeria as the Mesopotamian civ which isn't Babylon slot this time, in spite of Assyria having ruled the largest empire in history up to that point. And Austria, Venice and Huns haven't appeared, and I doubt they will as future DLC.
Iroquois is a possibility I would agree, but on the 'returning civ' rationale I think that Sitting Bull coming back (this time leading Sioux instead of 'Native Americans') is just as plausible. I can think of no reason why other groups like the Apache, Navajo, Comanche, etc., aren't at least just as likely to show up as the Shoshone. It's not like it saves them work since they would hardly going to copy their Civ 5 design anyway, and in terms of popularity yes Shoshone will be fairly popular now among civ fans for having appeared in Civ 5, but groups like the Apache already need no introduction.