I'm kind of leaning towards the final civ being the Berbers led by Tin Hinan at this point (maybe with a Tuareg Cavalry UU and Foggara UB?). I think that people may have become a little fixated on the idea that the final civ is likely to be Portugal or a Native American civ because the 'Assembling Typhoon' leaks suggested that Byzantium, Portugal, the Maya, Babylon or Assyria, Ethiopia and a Native American civ were likely for the third expansion pack. Although almost all of that did come true it's impossible for both of the last two predicted civs to get in the game now (unless there's a fourth expansion), and given that Firaxis know that we all knew about the leaks (as they also read the forums) I think they might have chosen something unexpected for their final civ. It would fit the 'patterns' of having 4 returning and 4 similar or new civs and having 3 female leaders per expansion as well as filling out a very empty part of the map. Plus, it would also mean they still have a few heavily requested/expected civs available for another civ pack/expansion.
Personally, I could see a potential Expansion #4 looking something like something like: Portugal (Joao II), Siam (Rama IV), the Navajo (Chief Manuelito), Argentina (Eva Peron), the Hittites (Puduhepa), the Ashanti (Yaa Asantewaa), the Kushans (Kanishka I) and the Noongar/Aborigines (Yagan) plus a two-civ leader like Constantine (Rome/Byzantium) or Taharqa (Egypt/Nubia)... or they could just go wild and try to get to 60 civs total with Bulgaria (Simeon I) and Haiti (Toussaint L'Ouverture). Either way that would give a good selection of civs from different time periods and different parts of the world, though admittedly with a larger proportion of 'new' civs than usual. We'll probably know if there is going to be a fourth expansion or not depending on whether or not a 'Civ VI Complete Edition' gets released after the last New Frontier Pass civ is available.