I thought about that too (more along the lines of "What if Columbus hadn't discovered the Americas"). I don't know much about the history of that region, but...
I did hear on the Discovery Channel once that Spain actually sent another group of troops to take Cortez back to Spain.. But, supposing something like that happened (and even if brief contact was made), this is what I think would have happened...
The Aztecs (and surrounding tribes/kingdoms) probably still would have suffered from smallpox. Also, other cultures would have jumped on the oppurtunity, and conquered the Aztecs. With the trade of guns and horses, the meso-american region would probably be thrown into a power vacuum. If they Aztecs were "first come first serve" (due to their gold), then their conquests would have expanded. (I'm not sure where to, though). But, with the trade of horses and guns for furs, many North American cultures would have flourished (Nanajo (even though they came about later) Souix, Illinois, Iroquois, and maybe some Northwest/Northern tribes later on).
I think the Aztecs (even if they did have decreased size due to desease) could have controled the entire Carrabien. (sort of like Rome and the Medditeranian). The Aztecs main rival might have been the Inca (I think they were around the same time as the Aztecs), which could have blocked, or stalled their entry into South America.