@KylluaZ mentioned this in another topic that was deleted, or something like this.
I always though the Aztecs should have extra experience for every fallen soldier, because of the (kind of violent) human sacrifices culture they have. Something like the Norse’s on Age of Mythology, that gets gods favor on battle.
So, my suggestion to improve them is to give XP for both enemies and their own soldiers that had fallen into battle. This would be realistic-ish to their culture that sacrifices their own population to appeal their gods. And also would gave new in-game life for they, because of the loop of going into battle, get experience points, send shipments to replenish the army quickly and going into battle again to get more experience for more shipments…
Also maybe adding some infinite cards of military units to help replenish the army over and over again would be nice. That would add a nice dynamic to a game with a Aztec player involved, with fighting breaking out all the time.