Here is the infamous "leak" re. Expansion 3:
https://forums.civfanatics.com/threads/all-quiet-on-the-civ-front.633171/page-32#post-15222899
Mentioned were Maya, Byzantium, Portugal, Assyria/Babylon, a Native American Civ, Ethiopia (plus presumably 2 more not named).
Byzantium, Babylon and Portugal are hugely likely.
My top picks for the other two would probably be Vietnam and (*crosses everything*) Italy but I can see that last slot going to another Asian or African Civ, or to Gran Colombia if they are adding another Native American Civ as well as a post-colonial one.