Boers, Mexico, and, I'm gonna throw in a wildcard city state, Singapore.
So, the Boers have a unique culture that can be about defending their vast farmlands and guerilla warfare.
Mexico has a long history and some fascinating culture to draw from. In a game about Empires, they actually were one as well, and would complete the North American trifecta. Additionally, Aztecs are not a replacement for this country.
So, comes to my bombshell Singapore. With GS, we are getting a lot more map interaction. Singapore is a city built on trade/commerce, one of the most important commercial centers in South-East Asia, and also has had to spend a LOT of effort on essentially expanding an island. Singapore could have some really cool game-changing mechanics in the engineering projects arena and be a different take on a Venice-like civ.