I'm very disappointed in the choice of adding Sioux over the Incas or Aztecs, but I do sort of understand why they did it. What I'm afraid of now though, is that neither the Aztecs or Incas will make the cut, at which point I'll have little or no interest in the expansion pack.
While Aztec and Inca cultures, architectures, warfare, system of god(s), languages, political systems, domesitcated crops and animals, methods of farming where all quite different, they didn't differ enough in one aspect, that being they were both empires with roughly similar technological levels that ended at roughly the same time (though there were rebellions).
I think when they ment 3 native civilizations that are as different as possible.. they ment:
Iroquois : Unified tribes, some agriculture, some hunting and gathering.
Sioux : Non unified tribes, no agriculture, all hunting and gather mostly from Buffalo.
Aztecs or Inca : Centralized Empire encompassing many different tribes (cultures, languages), large scale agriculture, animal domestication, supplemental hunting.
It's a shame though, I really wanted to see Aztecs and Incas finally depicted in a game as different as they actually where.. Maybe ES will the missing one if they make another expansion pack. Here's to hoping!
I think it might be becoming time to start writing up my own design doc for a Nations game..