The enlightenment idea was actually exactly what I was thinking, and I had the idea that "enlightened" enough Couatlinti transform into full-fledged Couatl, and are forevermore worshiped as demigods (in an ancestor worship sort of way) by the remaining "children".It seems like the Couatl are their own Yin/Yang. You've got a Serpent (a strong symbol of the earth) and the bird (one of the heavens) in one being. In a way, a part of the allure of the Couatl to the Couatlinti could be the Couatl's combination of these two forces. The Couatl could represent Enlightenment to their “children” by the fusion of Yin/Yang.
Because of this, finding some sort of spiritual opposite of the Couatl might be difficult. You could always reason that since the Couatlinti are so organized:
Naturally their nemesis would need to be something chaotic, or possibly mad. Congratulations! You have a perfectly legitimate, story/plot-based reason for having Cthulu in the setting!
Makes sense, actually. I suppose the Orcs closest to the "border" of Couatlinti/Orc lands would know the most about the "snake-men". Some of the more independent-minded and forward-thinking Orc kingdoms could have adopted Couatlinti weapons/armor and a bit of culture, perhaps. Those could be a good force for any PCs to "ally" with.You said it yourself: Slaves. Who are the Orcs enslaving if not Couatlinti? And as every group of people ruling another realizes, Orcs must have realized it's easier to control their Couatlinti slaves if they can speak to them, and so must have learned some of their language, at least enough to teach the Orc tongue to the Couatlinti slaves. After a while, some Couatlinti may have let a few facts by from time to time. The Orcs may not trust all they hear from their slaves, but what lies they're told can tell them just as much as the truth about the Couatlinti.
Bronze weaponry and occasional magical defenses could help, yes.Good idea. In H. Rider Haggard's “Virgin of the Sun”, a book that deals with the Inca fighting a Viking (I know, right?) the writer describes the Inca using bronze weapons. You could extend that to armor. It gives a better explanation for how the Orcs keep losing, anyhow.
Hmm... What if humans are from a land beyond the Dwarves (since the dwarves are cut off from most directions by ocean and a mountain range), and there is a small, but thriving human diaspora/refugee region within the Dwarven lands? It would give the image of the Jewish/Muslim diaspora that existed in medieval Spain that way.I wouldn't bother with humans at all. The Dwarves fill the European niche, and there really aren't a whole lot of other groups you could involve without losing the Conquistador/Mesoamerican vibe you've got going. There's also the added bonus of your campaign feeling like one of Tolkien's stories about the Wars of Moria where the Dwarves had epic battles against their foes. It lends a mysterious quality to the story.