Rather, it would be simpler to set it up that you could build multiple different improvements on the same node. IE you see horses, so you can build a stables like usual or a "farm". Might be interesting, would give the player some more modularity in meeting their needs. On the other hand, I feel people would end up with some "best strategy" that they would always use. In Magic the Gathering, the base rules are set up so as to never let any deck actually work as intended- thus keeping a automatic power check on win the game combos, etc. Such rules allow magic to have very lenient rules as to what a card can do.
Similarly in elemental, you get a sovereign that encourages certain strategies, and a resource base that pulls you towards certain approaches, and makes others difficult.
As such I think it's best just to put in rewards that justify strange situations such as all horses and no food, or tons of iron and no gold. Instead of allowing people to turn horses into food or iron into gold, wouldn't it be better to provide additional benefits for owning tons of horses or iron, that could be taken advantage of without alternative resources?
More specifically, I think the ideal answer would be to add in some more mount choices such as "heavy war steeds" that give an armor/attack bonus instead of a speed/move bonus. (In the past, some cavalry were indeed used to flank and outmaneuver, but they were often just loaded with additional equipment, and used as a sort of "tank" that were hardly any faster even in tactical situations, and often slower in strategic ones.) Chariots could also be represented, etc.
This way you wouldn't need to level up your cities, and could instead play like mongols, taking advantage of your better troops to steal what you need from your rivals.