I'd be happy with a system where food wasn't the determining factor to growth (it was needed, but not determining).
Then when a city has excess food (determined by its food consumption rate, which is determined by policy and growth), that excess food gets shoved onto the trade network.
Depending on technology level and transportation network, a larger or smaller percentage is lost. Then the destination city, also depending on its technology level and transportation network, another percentage is lost.
Suppose 70% lossage on export on a land-locked city, and 70% lossage on import on a land-locked city, in the ancient era. Then 90% of food doesn't make it to the destination.
Having river access would reduce the lossage to 65%, and ocean access to 60%, which reduces the overall lossage to 84% (a 50% improvement).
By the modern era, you could have as little as 10% lossage on export and import, making a bread-basket city 80% as good as being right there.
You could then make deals with foreign powers to buy or sell food -- with the note that if you can make someone dependent on your food exports, then blockade them, they end up with serious unrest!
For a long time, I have advocated a similar idea.
Basically, I think food and hammers should be transferrable among cities, subject to certain limitation of course. That way, cities with surplus food/hammers can transfer some of the excess production to cities that need them.
The specific implementation of this idea will go something like this.
There is going to be a screen divided into three sections.
The left side of the screen will show a list of cities and their corresponding food/hammer production amount. From this screen, we can pick cities, their particular production (hammers or food or both), and the amount of that production that is to be contributed to transfer to other cities.
For example, the left screen will show the following info:
City A - 10 food/16 hammers
City B - 3 food/9 hammers
City C - 4 food/8 hammers
From City A, I have it contribute 2 food/4 hammers for transfer to other cities. From City B, I have it contribute 1 food/1 hammer. I ask nothing of City C for contribution.
The center of the screen will show the total amount of food and hammers being contributed to inter cities transfer. Continuing the above example, the center screen will show 3 food and 5 hammers availabe for transfer to other cities.
The right side of the screen is where I can select cities that will recieve the transfer. I designate City C as the recipients of the 3 food and 5 hammers.
There will be limiting factors of course. Maintenace cost increases with size of transfer. Maintenance cost is 1 gold, for example, for every 3 food transferred, and 2 gold for every 6 food transferred. The numbers I used are arbitrary. I'll let the designers determine the exact numbers.
As a non-sequitor, I like to see the ability to exploit landtiles that are not within any city raidus. We should have the ability to get settlers to build a village on one of those landtiles outside of any city raidus but within the empire boarder. The village will produce hammers and food. These hammers and food will automatically go onto the center screen which I have previously said will show the available quantities of food/hammers available for transfer to other cities.
Continuing the above example, the center screen showed 3 food and 5 hammers. Lets say I build a village and now this village produces 2 food and 1 hammer. The center screen will now show 5 food and 6 hammers.
Since maintenace cost increases with size of food/hammer transfer, players will not abuse the village feature since the more village they built, the greater the amount of production transfer taking place, which will eventually bite into the gold revenue. We can also make it quite costly to built a village. For example, a combo of 1 settler and 2 workers are needed for a village to be built.