Main question is IMO not geographical spreading or what continent is in need of more representation, but what civs are romanticized enough to make people think "Yea, I'd conquer the world as you!" That's the problem with most sub-Sahara African civs and even more for South-American civ. They need some characteristic or appeal that makes their UA and UUs interesting. Moreover, the civ needs to add something to the game. The Zulu's seem a likely candidate due their history in the civ series, but are another war-civ. Same goes for Poland, which could add some very nice units (Polish lancers or Winged Hussars), but we have enough unique cavalry units already. The number of trade civs is low; we had Arabia, which is joined by the Netherlands. Mali could make a return? Maybe it can become a trade/city state focused civ.
Brazil or Grand Colombia make little sense IMO as they are very new states, not really civilizations. (Yea, same goes for America, but hey, the game is made there and it forms half of the audience)