Basically, the designers of Maztica set out to create something that was very much like the real world version of the Americas in the 15th century.
This sounds like a pretty good idea until you consider how unlike the real world Europe of the 13th century you average fantasy campaign setting actually is. Even if the inspiration for consensus D&D is European Medieval the execution tends not to be Europe as it might have been, but rather Europe as it might have been if everything mythic was ordinary. The consensus D&D reality literally teems with mythic creatures and is awash with magic. So, by comparison, Maztica emphasis on only slightly changed history seems positively mundane and empty.
The correct translation of the Mayan/Toltec/Incan/Aztec/Hopi/Adena/Five Nations/Tlinget/Inuit/etc. cultural environment would have assumed not only that everything fantastic was real, but that everything fantastic was ubiquitous. Like the version of 'Europe' that rules traditional D&D, the cultures of a well considered American setting would be less insular, more prosperous, more travelled, would have an established and well known 'common' trade tongue, and would be allowed virtually any technology that didn't interfere with the feel of the setting (gunpowder in medieval Europe, iron in 'medieval' America). In addition to every myth of the native people's being true, it's worth making most every myth held by the Europeans about the native people's be true - there really would be lost cities of gold, lost tribes of Europeans ('Norse', 'Egyptians','Irish','Chinese',whatever), fountains of youth, etc. - and you should incorporate these 'secrets' in way that would fit them into the myths of the native cultures about themselves.
Plus, it's worth considering how the fantastic things from the rest of the world might have leaked into the new world, had those fantastic things existed, and how their presence might have impacted the myths of the New World. Real dragons almost certainly would live in the new world. In some cases might they not be viewed as Gods? Are their city states in Maztica ruled by dragons? Are their ice loving Harpies amongst the 'Inuit' people? And so forth.