Ok, the one I was thinking are a lot more boring that the one before but there it´s goes, Give the the Mayan culture in his zenith (circa 700-900 C.E.) the knowledge to make
Terra preta, as his fall are most likely associated with his agricultural practices, as I subscribe to the Theory that his agricultural practices were insensible in the poor jungle soil that was the base of his agriculture, the Cyclical Drought of the region could be worked with a well tough series of waterworks and reservoirs, as historical the Mayan do, But the thin and poor soil of the jungle could not be worked around, Unless you use the Terra Preta, terra preta is man made so there is no reason that Mayan couldn´t developed this soil, or import it from the .
As the Mayan Civilization lasted as a force to be recognized in mesoamerica until 1690 aprox, that was when the Spanish could conquest his last city, i support the Mayan civilization.
So here you have a civilization of some 15 to 30 millions peoples( the
new estimate of the place in his zenith), in a moment that Spain itself have no more than 8.550.000 people, in the mesoamerican basin, that even with the worst estimates of the old world disease exchange (98-99% of the pre-columbian population) still give you some 150.000 to 600.000 people to continue and rebuild the civilization and oppose the Spanish rule.