Some more points I'd like to add to the conversation:
- If the Maya have a much higher population, and are establishing colonies as far south as Panama, might they start colonies also in nearby Cuba? How far will Mayan traders go? Could they reach Florida? Colombia? How many cultures will be affected? I'm thinking mainly of the Taino... they are pretty close to the Mayans and could pick up a couple things after centuries of contact. That would change the progress of Spanish colonization right from the get-go.
- If there is some kind of butterfly net surrounding Mesoamerica and Spanish colonization follows as OTL in the Antilles, it's only going to take until 1501 with Rodrigo de Bastidas or 1502 with Christopher Columbus for the Spanish to start poking around Panama. They should run into some Mayan colonists which would pique interest in the mainland thereafter.
- What will population figures be like in the Valley of Mexico? Without the collapse and with the introduction of the potato, population is going to be booming. However, this was already one of the most heavily populated areas of the world when Cortes entered the valley... could it really be even more densely populated? Even though I think Mesoamerica could reach a population max of 60 million instead of 24 million with the potato, guinea pig and quinoa... that's just a lot of people to put in one place. I would think a lot of that population is going to be diverted into other highland cities that were in OTL much smaller.
Still, what's interesting is that Teotihuacan will no doubt have a vested interest in being the biggest city on Lake Mexico. It only reached 125,000 in OTL at its height. Tenochtitlan reached 215,000. If Tenochtitlan is founded several centuries early, its going to hit that figure pretty quickly, which would be bad news for Teotihuacan. I'd imagine that some laws and statutes will be introduced to keep Tenochtitlan from developing so extensively, limiting their population to at most half of Teotihuacan's. That extra population could be diverted to Teotihuacan instead.
My recommendation is to have 1.5 million people in the Valley of Mexico instead of 1 million. Teotihuacan would have 250,000 of that, double their OTL height. Tenochtitlan would have 125,000. Texcoco would have 100,000 instead of 24,000. Tlacopan, Chalco, Xochimilco, Otampan and the others would also be much larger. As such it would be much harder to lay siege to this area if more of the population was diverted to the shoreline of Lake Mexico than the island of Tenochtitlan... in fact, it could be nigh impossible to starve out the populace... you couldn't bottle them up on the lake, you'd have to take all of the mountain passes instead! The city would be more dependent on the potato crop coming from the western highlands and around the valley than on gulf coast crops and fish, so if the Spanish take the coastline the Mesoamericans wouldn't be as impacted as in OTL. Quick Spanish conquest would be that much harder to achieve.