My thoughts are -- if Paradox takes the opportunity to put some thought into how the religious and political peculiarities of the Aztec Empire can be simulated, more power to them! It will make CK2 more flexible for modders, and maybe benefit future titles. (Although the original Aztec Empire wasn't as much a territorial empire as a city-state federation with tributaries. Clearly this is a much-evolved Aztec Empire,but for color I think there should definitely be a decision where your state supplies captives as the price of survival.)
A semi-plausible alternate history explanation would be worthwhile. Those ships look awfully Norse -- maybe Vinland thrived long enough to pass on a bit of technology, and immunity to some European diseases? Eventually I had wanted to add to my alternate history M&B mod, Empires of Faith, a New World that had seen low-level coastal colonization starting in the 1100s, with native empires given enough time to recover from disease without having to do with the one-two punch of epidemic + conquerors.
My only reservation with Sunset Invasion is that even a worth product on its own terms can dilute a brand somewhat -- like Laphroaig doing a line of energy drinks. It can also subtly change the institutional culture of a company focused on artisanal products, like EU and its descendants. But presumably Paradox's execs and its marketing folks have already given this some thought.