You forgot, ironically, to mention the Incas, and I suppose they were not “different” enough from the Mesoamericans and that is probably because to this day they share the architectural style of the Mesoamericans. Hopefully it is possible that in the near future a unique medieval South American architecture will be developed for AoE 2, hopefully one day they will also add some extra South American civilization such as the Chimú *
Inca architecture
Chimú architecture
Wari architecture
Chachapoyas architecture
*The Chimú theme created by the user @JCC3001 is very complete and it must be recognized that despite the language barrier, he managed to do a whole research work with data that I didn’t even know (it’s not because I’m presumptuous, it’s because my teachers taught me these topics in school).