The Aztec empire being the dominant force on the continent could be a very unfortunate situation for everyone living there.
They were so unpopular with every surrounding nation (because of all the murdering and kidnapping and human sacrificing) that when the Spanish showed up, the vast vast majority of the soldiers that fought against the Aztecs were from the local peoples.
If they'd endured as independent groups into the 21st century without being colonised by Europeans, as the map shows, they would almost certainly have developed defined borders.