The past 2 expansions have either purposely, or coincidentally, followed a pattern so that's what I have been looking at.
8 new Civs and 1 alt leader.
3 female leaders and 6 male leaders.
4 returning Civs and 4 'new" ones. "New" as in name hasn't been used even though elements of the Civ has been used in the past.
At least one of the returning and new ones have been European. Georgia might be the exception.
R&F: 3 female leaders were Seondeock, Wilhelmina, and Tamar with the "new" civs being Cree, Scotland, Georgia, Mapuche. Returning European civ is the Netherlands while Scotland is the new one.
GS: 3 female leaders were Kristina, Dido, and Eleanor with the "new" civs being Hungary, Maori, Canada, and Phoenicia. Returning European civ is Sweden while Hungary is the new one.
Other factors:
One new South America Civ each one: Mapuche and Inca. Of course SA was bare to begin with besides Brazil but I expect another one to appear.
African Civ: The past 2 have been returning so we might get another returning one, though they could always throw out a new one.
North America has got one each in Cree and Canada. I think it's safe to say that any new NA would be native. I'm not counting Mesoamerica in this.
Nothing from East Asia in GS after getting several in R&F. I think we'll get one more.
So In short I could see something like this:
Returning:
Maya (given), Portugal (returning European), Assyria or Babylon (another ancient Civ), Ethiopia (most likely) or Byzantium (if they consider it Middle Eastern and not returning European
)
New:
Vietnam (from East Asia), Colombia or Argentina (from SA), Italy (New European and the largest gaming market not covered), Navajo/Apache/Comanche etc. (New NA tribe geographically from the SW USA to fill out the map.
Alternatively Ethiopia can be renamed "Axum" to be one of the "new" Civs as well. I could see then Colombia or Argentina not being in for Axum and therefore put Byzantium as one of the returning one.
For the three female leaders I would go with obviously Lady Six Sky for the Maya, Trung Trac for Vietnam and then it would be a toss up between if Byzantines or Argentina made it in with Theodora or Eva Peron respectively. I think Colombia with Simon Bolivar is much more likely to appear though than Argentina.