Since Ehtiopia uses the most modern leader of all available in Civ 5, then Brazil under Pedro II or Gran Colombia under Bolivar make much more sense - especially to fill out South America. They would be much more a representation of a certain state or country rather than a civilization, though. Gran Colombia in particular, as it was (and is via its successor states) a part of Latin American culture and civilization group, which is still strongly tied to Spain (unlike the US, whose cultural connection to England/France/Germany has loosened up a bit since the 1700s). Brazil developed a more unique culture, and therefore could be the last mystery civ, but I doubt it - it would be stupid then not to include Portugal. And as we know, Lisbon is just a city-state now (but it might be a sign that a Portugal DLC will arrive some time)
After all the PAX news I'm still guessing the mystery civ will be African or Asian. Most likely Kongo, the Zulu, or an Indonesian civ (I mean the region, not the modern state). What I wish it would be, then Lithuania (unique religious playstyle - maybe prolonged immunity to conversion) or Sumer. I've ruled out Hungary and Poland, because they have remained as city-states in the lates coverages, but as we know, it's easy to swap a CS name with a new one to make place for a full civ.