I think the most unique or special aspect of the Aztec in comparison to the other groups around them was that they literally settled in Lake Texcoco on swamplands and reclaimed land from Lake Texcoco, both for farming (chinampas), and to build their capital city, Tenochtitlan. This city was built largely out of stonework, and is most commonly estimated to have a peak population of over 200,000, meaning it was larger and more grand than any European city other than perhaps Constantinople, at the time.
In my opinion these are the most unique and interesting features of the Aztec: their reclamation of land and their construction of such a significant city (and over a period of only about 200 years). I don't think that these are well-enough represented in Civ 4 or this mod, especially as the tiles around Tenochtitlan don't really help making it a large city, even with the flood plains and food bonuses.
I don't really have any specific suggestions so perhaps I should've posted this in the general discussion, but this thread seems to get more traffic and interesting debate/ideas, so maybe people will have something useful to say in response?
Also I've been a fan of this mod for some time now but only recently made a civ fanatics account, and just wanted to say I think Leoreth and the community are really great and have made an amazing mod!

I look forward to having more to say here in the future