Some minutes ago I watching Liereyy's stream and he was playing a fortress game. In that game, he chose not to finish a farm in Dark Age so he could take advantage of horse collar. So I thought, why not change Incas team bonus to "farms already built are affected by farms upgrades"? It would be a useful improvement from the current bonus, not something broken and it could be some nice compensation for the loss of their vill armour and attack bonus.