While I'm extremely hesitant to suggest anything along the lines of new economic goods after the last attempted implementation, I do think there may be a point about rural lords not needing to trade for the survival of the region like cities do. Perhaps to sustain food production, non-city regions need to buy some form of good from cities? Fertilizer? Irrigation specialists? Sickles? Blacksmiths? All of these seem silly though. If the region *doesn't* get this good from cities, then their food production doesn't go higher than what is sufficient for the region to feed itself, meaning a city will starve if it doesn't sell enough of these goods to their surrounding regions.
I have had similar thoughts in the past. If the rural isn't selling food, then why is it producing food? Who's going to grow 10,000 bushels of food that just sit and rot in the warehouse, and then get burned because they're spoiled?
But let's say you do implement something where you have a feedback mechanism from the city to the rural that the rural needs to keep food production high. So, the rural stops selling food to the city, so the city stops selling widgets to the rural. So what? Why does the rural lord care? He's not selling the food to the city, so he doesn't need to still grow it. His region is fed, and he gets gold from taxes, so why does he need the food that he's not selling anyway?
There is an imbalance in the food system. The city needs what the rural has, but the rural doesn't need what the city has. The only way the city can force the rural to give up the goods is by getting the realm leadership on their side. And the realm council has to be willing to stand up for the margraves, and tighten the screws on the lords refusing to sell. That's the only real power the margrave has to enforce the food sales on the lords.
There really is no mechanism I can see that allows us to give a strong need for the rurals to sell food to the cities, if the rural lord decides that he doesn't need the extra gold. The only way I can see that enforcement coming down is via IG enforcement by the characters in the realm.