Currently, trade exists to facilitate the exchange of food. Food is justified, because it makes cities dependent on rural regions, which counterbalances the importance of cities. Without food, cities would be self-sufficient regions, capable of supporting all economic structures, potent fortifications and able to produce large amounts of gold. That would be detrimental to gameplay, hence why food exists.
What you are asking, is the introduction of a new resource, not for the sake of balance, but for the sake of trade itself. As I demonstrated, trade is a supplementary feature, the role of which is to facilitate the exchange of food. Trade currently fulfils this role, fully, and so there is no point in adding to it.
