Inca is all but confirmed I feel, and with good reason. But given TSL considerations, I think it's unlikely we'll get another Andean civ. I think Patagonian is a stretch a well given the greater potential in civs in the north of the continent. I hope Brazil doesn't get the slot, presuming there's only one other. The Muisca would be fantastic, if a little close for comfort with the inca for TSL. The Tairona would be cool as well, and a little more distant.
For something a little more off the wall they could go for Arawak / Taino and bring a civ that covers north coast of South America and the Caribbean for the first time!
Presuming they don't go for Brazil, I would bet on te muisca though, given the strength of mod love for them in V and I mean, their leader would be freaking EL DORADO, a man covered in gold. What's not to love?