Well there were the Mississippian and Mound Builder cultures in the Mississippi valley and southeast US. They were not precisely empires(that we know of from the archeology) but did have large cities rivaling Mesoamerican cities.
You could argue that the Iroquois Confederacy was an empire like the Aztecs, only instead of a triple alliance, the Iroquois were a 5 way alliance that exerted power down to Virginia and north in to Canada and west in to Ohio.
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