The Mayans had already declined by the time of the Spanish incursion. Peru was in the midst of a civil war when they showed up there and would have been split up into smaller kingdoms by the end of the century as we count it. The Aztecs while still advancing and expanding their empire, Montezuma was showing the first signs of decay through being a weak leader. This could have lead to strife and possible rebellion of vassal states with any number of other catalysts besides the arrival of a handful of soldiers with superior weaponry.
On the positive side for this to happen, there were deep sea going fishing villages along the Atlantic. If someone had thought of exploration for the Aztec empire's number one need, sacrifices they would have gone south.
The Caribbean wasn't always a friendly place. While easy to trade with, they also had some formidable warriors. The Aztecs could not have floated a big enough flotilla to conquer them, or even raid them for a worthwhile amount of captives.
They were already familiar with land routes north and knew more or less what to expect. The Apaches, Navajos and Hopi have already had negative encounters with the Aztecs before 1492.
I don't know if the Aztecs were aware of the Maya. By 1492 the Mayans were reduced to small farming enclaves in the Jungle. If the Aztecs were aware of them they might not have found them important enough to mention, being they had control of all the important civilized nations of the area.
I thing there must have been some kind of contact between Mezoamerica and The Andes, maybe earlier during the more advanced Toltec and Olmec empires just from the fact that they both built similar pyramids and while the Aztecs were bloodthirsty killers and the Incas were a little more mild on the sacrificial side they both did it the same way.