The official expansion page refers to a "Native Americans" civ... whether or not that will be officially named "Sioux" or not isn't a given. I could see them grouping Amerindians...
With the 'beyond the sword' approach, the unknown Civs are more likely to be along the lines of a 'Poland' than a 'Maya' -- so expect more European Civs, especially since they will be used in gunpowder-timeline war scenarios that dominate European lands.
With the 'beyond the sword' approach, the unknown Civs are more likely to be along the lines of a 'Poland' than a 'Maya' -- so expect more European Civs, especially since they will be used in gunpowder-timeline war scenarios that dominate European lands.