Since the Spanish had a very big advantage over the indigenous, it is likely that someone else than Cortez would have conquered them instead, or that the English or French would.
But if the Aztecs had kept their power, it would most likely be a country today with Mixed Nahuatl-Spanish speaking country (due to the influence of Spanish on the countries next to it), which would however likely converted to a form of Christianity, due to the influence of the neighboring countries.