Our current trading system doesn't work because there's little incentive to trade. There's food, and there's gold. Most realms have an amount that is close to break-even, and won't trade with others. The realms that do have a surplus aren't likely to sell much of it in case of another Great Drought (or war, or regular droughts, or simply because they don't like the price).
We need to add Metal. Once lords need to buy metal to outfit the recruits from their recruitment centers, or for their smithies, then we have more trade dynamics. Lords will be able to post food for metal trades, metal for food, metal for gold, gold for food. Traders will be able to combine a metal to food sale with a food for gold sale and a gold for metal sale. Mountain regions like the Divide range will suddenly become more valuable, and war for those resources will ensue--or surrounding regions will be bought off with favorable trade agreements.
Later, we can add wood from our woodlands. Wood is consumed daily in cities and townslands to cook food; it's also needed to build palisades, construct siege engines, and ships (yes, those are on the to-do list still).
The problem with Trade is the gold is boring. We all have gold. But metal and wood would provide a new incentive to trade.
All trades should stay, resource for gold, or gold for resource for simplicity, IMO. Also, I believe things have been considered like this but the key thing that was decided was not to make it necessary, but beneficial. Necessary isn't going to help the atmosphere towards trading as you are adding penalties, not benefits, which generally works a lot better.
