If it is not Portugal, I'd wager the last Civ will be a new entry to the franchise and led by a woman, as this will complete the '4New/4Old/1Alt, 6Men/3Women' patterns of yore.
Taino (Anacaona) we can scrap. Caguana's addition to the game in NFP discounts them. Haudenosaunee (Jigonsahsee) are also unlikely, since they are were previously present under their popular name, Iroquois. I don't think they would count as 'new', even if added under 'Haudenosaunee'.
This leaves a couple of options:
> Tonga (Queen Salote)
> Madagascar (Ranavalona)
> Italy (Matilda di Canossa)
> Apache (Goyuen)
> Cherokee (Nanyehi)
> Berbers (Tin Hinan, Sayyida al-Hurrah or Dihya)
Some options I didn't mention because I don't think they're worth listing. Ukraine (Olga of Kyiv) won't be added in an NFP that already adds Byzantium, and I doubt Ashanti (Yaa Assantewaa), Benin (Idia) and Hausa (Amina) would be added with Ethiopia already present. Likewise, Gran Colombia's addition debunks any options for Argentina. (Eva Peron)
Out of these options, I think Italy and Cherokee are the most likely, in terms of leader worthiness and recognizable Civ names. Berbers is an idea that mostly exists inside our CivFanatics bubble, and I doubt they would impress the casual fans. They're better off being saved for Civilization 7, where they can be an expansion addition.