Alt-Aztec

The El Guapo of Mesoamerica was that the local hegemon, Tenochtitlan, didn't control down to the Spanish Caribbean in time. In a generation it'd have entered the Bronze Age and assuredly would have integrated the Nahua / Otomí heartland better than it had. I propose, though, that we already own the contrafactual, for "Columbus 1522" and "Cortes 1549". Spoiler: it ends the same.

The Tlatoani system was like the Inca system at heart. The emperor was polygynous and ruled, in large part, through a network of direct relatives. Motecuzoma II, granted two more decades, might well have fended off some initial Spanish raids, which he couldn't given his poor grasp on the coast 1519. But then he'd have been older. And the smallpox would have come - come faster, in fact, given the better roads in this contrafactual. I give a coin-flip that the tlatoani survives this.

Then there would have come questions about the succession. Cuauhtémoc, as competent an Aztec as any, survived smallpox in his twenties in our world; I'll allow he'd have survived it in his forties in the alt'. But would Cuauhtémoc have been the only survivor? and: would he have been at the capitol when the plague hit? In the contrafactual a whole generation of Monty pups are roaming the city. Several cities.

Yes, that's the story of the War of the Inca Brothers. Atahuallpa was in full command of the northern suyú when the Spaniards came. That wasn't his problem. His problem was that he'd had to conquer his way there, through already-conquered peoples.

The Incas had the problem of Elite Overproduction, the same problem we got - except that we're not as inbred about it. The Aztecs will have needed to solve this problem too and I'm not seeing that Monty had it in him.