What if the Aztecs defeated the Spanish and went back overseas in their boats?
the Spanish would come back, but by then...
1. The Aztecs would've gained some level of immunity to disease, because obviously in this alternative history they survived the first attack.
2. They would've been more smarter, and probably institute different changes to their military and political system to adapt to the threat.
3. They could've even stolen and learned to use a couple of the Spanish weapons, not that it would've helped - they could easily defeat the Spanish with lesser weaponry.
4. Meaning that an Aztec state could've survived in Spain.
5. Meaning that possibly the very future of America couold be different.
6. Meaning that perhaps even thuogh America could've been born, there would've been no Mexican war, which would've meant that the Civil War would have happened sooner because there wouldn't be more new territories to or not to put slaves in, meaning that the entire history of the United States would be quite radically different.
7. Meaning that the Spanish wouldn't have gotten as rich as they could, and European history would've thus been altered. Perhaps a different kind of World Wars?
8. Meaning that possibly East Asian (i.e. Chinese) contact with the "Pre-Columbian" empires could have surely happened, thuogh of course not a huge fleet, meaning... that perhaps the Chinese, because of their "ALL ARE BARBS! LETS SINICIZE THEM!" mentality, would've tried to make the Aztecs more sinicized. meaning...
9. Today, some things would be "Made in Aztecland" and they would contain tons of lead

or human blood.