I noticed that several respondents have brought up butterflies. You do need to think about these, but there is no reason you can't reasonably speculate on how a more successful Mexican resistance to the Spanish invasion could affect the rise and fall of European powers and the colonization of the new world. I will offer a few ideas of my own:
1. To prevent formation a Spanish empire based on the conquest and plunder of ancient American civilizations, you will probably need a point of divergence sometime before the European discovery of the New World. Once Spain is established in Cuba, with expeditions reaching the American mainland, they will soon learn of MesoAmerican civilizations with wealth and seek to reach them. Even without Cortez's unlicensed conquest, Spanish domination of Mexico and Central America is pretty much inevitable - perhaps not outright conquest, but the end result would be about the same.
2. Don't forget Peru. Spain is still going to explore south and west and eventually encounter the Incas. Even if Mexico never falls to the Spanish (very unlikely) Peru will provide the Spanish with lots of gold and silver to plunder.
3. A Spanish Empire NOT based on the quick fix of plundered wealth might be a better and more successful empire anyway. If Spain reaches the high civilizations of Mexico and Peru and, for some reason, is unable to conquer them outright the Mexicans and Peruvians will still be within Spain's orbit, and provide much better trade opportunities than the other areas of the Americas.
4. You need to find a way to eliminate or reduce the impact of European diseases on native populations to radically alter the course of Native history in the New World.
1. A delayed conquest of Mexico would likely lead to the Aztecs collapsing into a civil war, which paradoxically makes it harder for the Spaniards to conquer them- it's easier to lop of the head and move in to the king's palace than to deal with a bunch of feuding city states. Just as importantly this will have a knock on effect for the Inca- who are the most likely of any of the Pre-Colombian empires to survive independent due to their isolation and relative unity. Pizarro was IIRC part of Cortez' expedition. Without this radical success the idea that you could waltz into the Andes with a few hundred men and end up a de facto king won't be as prevalent.
2. Again this assumes they conquer outright. Most of these conquests were not driven by the state but rather by ambitious second sons and the like who had the drive to climb up in the world. The inca are strong and distant, if they hadn't been hit right after a civil war Pizarro wouldn't have gotten as far as he did.
3. Exactly- up until the 1600s the Netherlands were more important anyway. And that's ignoring the effect of hyperinflation. Just as important is the role New World Silver had in China- the Ming's economy was even more devastated by the influx of silver, and also allowed Spanish merchants to trade silver for silks, porcelain etc. That trade vanishes with a delayed conquest.
4. The disease was a result of many things- but with a delayed contact/conquest, the effects of social collapse caused by war and exploitation, together with fewer disease vectors, likely takes some of the sting off the disease, how much is almost impossible to predict. Say 10-20% fewer dead in Mexico and 50% less in the Andes as minimum and maximums.
Just as important is that these were the total deaths- over several decades- and didn't necessarily happen all at once. Again the civil war/diseases will likely kill at least half of Mexico but it will take time, and the Spaniards may or may not be in a position to exploit that.