I'll use my intuition to say again that I think he was intentional and the Aztecs won't be the third civ.
They might be more fun to play in this game than the last game, but I just can't see how the Aztecs can make an appearance in the third act, which occurs after the 1860's. By that time, the Aztecs are gone. Most of them have become mestizo. In fact, I don't think there exists any full-blooded Aztecs.
If you look at the pattern of the acts, however, it would make sense that the story begins on the East Coast and works its way across North America. They could go from the East Coast to the Great Plains and down to Sonora, but the Mexican Independence occured in 1810, so that doesn't fit the timeline, and even if it did, the Aztecs and their low-tech weapons would have little to do with the war.
The storyline could somehow take the Black family to Peru, where the final act could illustrate, not Peru's independence, but their final recognization of independence in 1879 from Spain, and the act could move on to illustrate the War of the Pacific. However, that act would have to illustrate the Incas having gunpowder weapons, which is not how they're supposed to be.
So with that said, the final act could always be a flash back to explain an event that ties the Black family with the first two acts and answer some questions that weren't answered in those acts. Who knows.
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