Mesoa is the (official?) Aztec/Mayan (Meso-American) area. It's not yet ready for individual buildings but help is always welcome, just let me know if you want to help. There are signs which show the way to relevant areas from the portal building.
Current people working on Mesoa.
The Temple - Currently half built, the main pyramid which is the cornerstone of the city, which can't be built until this is finished.
Mesoa City - Once the temple is finished, on the other side of it, will be the city, with quintessential triangular Mayan buildings and possibly a sand/sandstone floor.
The Village - A tribal village with wooden huts and a dirt floor and surrounded by dense forest (the village floor needs to be done before anything can be built).
Sand Flats - If you've seen Apocalypto think of the area before the city, where all the workers are and some slaves are sold instead of being taken to the main city to be sacrificed.
The Death Pit - An area attached to the main temple where the remaining slaves are set free, but first must avoid being shot at with arrows and spears from the city hunters. Immediately after the run is a pit where are the bodies are thrown, surrounded by wheat.
The Docks - Dock area with Aztec boats, and perhaps a train station nearby.
Portal possibly needs to be connected to Tron City.