I remember reading somewhere that there was a King in the Mexico valley who was something of a philosopher, and that he flirted with the idea of monotheism. What if this caught on and such.
Well, I think the big change would be if human sacrifices are abolished (which I think this king proposed). Without human sacrifices, the empire that eventually rules Mesoamerica will be stronger, as they wouldn't need to have independent states within their territory with whom they can go to war in order to get captives (IOTL, one of these states, Tlaxcala, decisevely helped Cortez in his campaign). So, the Spanish will have to deal with a stronger empire.
However, they will probably still succeed, eventually, as they had many advantages. But the outcry against the conquest would be even greater than IOTL.
IOTL, priests like Montesinos and De las Casas loudly protested against what was being doing, even putting into question the right Spain had to conquer these lands. They weren't heard, but the sense of guilt and ilegitimacy of the conquests was soehow adopted by the Spanish in America, and survives even to this days. By reading Spanish chronicles, you'll get the idea that somehow, most Spanish conquerors always knew there was something wrong in their acts. This could be translated in better legislation concerning the treatment of Indians, for example, as they won't be able to use the excuse that Mesoamericans worshiped demons and sacrifeced victims to the Devil.
Also, I'm sure somebody would claim this king was a saint who had left the old world and had been preaching Christianity.
Finally, Spain and the rest of Europe will have a different image of Mesoamericans, more akin to the one they had of the Incas.