The campaign extends from the High Medieval Period to the very early Age of Discovery. The Aztecs are in because of Hernan Cortes. I propose a new acronym for this game, M2RAoD:TW, or Medieval II Renaissance Age of Discovery: Total War. I think this would be too much work to do for a couple dozen turns at the end of the game. Seeing as they already used the Mongols, guess what the expansion's going to be.
I'm far more concerned with why, out of all that is righteous and good in this world, Spain is a single faction.