I would love to see the Mapuche, but lets be honest, most of the world has never heard of them, I think modern nations like Gran Colombia and Brazil have got a better shot at filling South America, and there's rumors of coffee being a new resource (Both Colombia and Brazil are massive Coffee producers)
Both areas have rich history, culture, and been involved in many conflicts. The Cristo Redentor is a wonder in Civ V, and Rio de Janeiro is a CS already, its not really that far fetched to consider Brazil into the game, besides if Portugal is not going to be in, Brazil could get us a portuguese speaking leader, without crowding Europe more, and filling south america the same time.
But I have to be honest, just imagining Bolivar in a Civ game makes my inner fan go crazy.
Anyway, I guess we could decide it by whom could get its own DLC scenario, and I think Gran Colombia with Simon Bolivar could just as well get its own revolution against Spain scenario later on, either play an American civ or try to hold your empire together as Spain. Or something along those lines.
So, my vote goes Brazil, with lots of hope for a Gran Colombia DLC.