Hmm... I really dont know how
I will try:
You have 3 Cities:
A B and
C. They are all connected, but imagine you have almost no techs so
A has only a Traderoute to
B.
Lets say
A has an Iron Ressource in his vicinity. You build a normal mine improvement on it and connect it with a road (or river or whatever) to
A. Now, instead of having Ressource-Iron in
A B and
C, you would gain one "Iron-Point" per turn in
A because you have a mine (like a Bandits Hideout produce +10 Crime-Points per turn). If you build an Iron-Smith in that city it will reduce Iron-Points by 1 per turn, like a courthouse reduces Crime-Pointsper turn. It also provides 1 Iron-Ingot per turn (or armour or tools, whatever you like).
Now all cities that have a traderoute from
A (in this case only
B) recive lets say 0.5 Iron-Points per turn. If you had more cities and multiple Iron-Ressouces, this will become an even higher number. Now Replace the Iron-Ressource with ALL ressources.
You got the idea? :/
You can add that normal Mines provide 1 point per turn, shaft mine 2, modern mines 3 and so on, maybe even link it to Techs.
You can add Storage Buldings, so if a city gets 2 Wheat-Points per turn but only uses 1 per turn it will accumulate points. With a storage-pit you can have at most 30 points, with a granary 100 points and so on...
Units could consume Ressource-Points when you build them: A Knight may consume 2 Iron-Points and 2 Horse-Points (abrasion while training or food
).
If a city hasnt enough ressource-points it just stop producing until there are enough points available again.
Only thing that I dont like now on this is that you magically generate more Points (in total) when you increase the number of traderoutes...
I dont know if it will cause performance issues, but since crime and pollution also has a diffusion-rate, this shouldnt be the problem...
edit: Just some more thoughts about it:
You can say a Mine produce one Point per turn in that city and one pointper turn which is devided by the number of traderoutes, so the more traderoutes a city has, the less points each of the trade-cities gets.
Also, you could construct an Oil-Truck unit for example, which would cost Hammers and also 100 Oil-Points. Once it is built it works like a karavan and can be brought to an other city. In this city it can be consumed to increase the Oil-Points of this city by 100. You could restrict those Units to a certain route-type. So maybe a early game "Cheese trader" can transport 10 Points of Cheese while a modern cheese truck (only moveable on highways and above" can transport 100 Points. THis way you would have a "restriction" of ressource trading by route-types.
Another thought: A SUpermarket and similar could consum Milk, Cheese, etc depedent on the Population of the city. Ex one point of each for every 10 Pop-Points. If the city has not enough cheese, the city will get a little bit unhappy about it.
Units could consume Oil-Points if they are moved, so warfare becomes more strategic.
This system would need a lot of balance... I dont want to force a player to spend 50% of their turn with sending around trading units. Thats why I recommend a natural spread of ressources. However, if you have "extreme" cities, for Ex a very big city, you willl have to transport food-ressources to it, if the city produces a lot of units you will have to bring a lot of armour and weapons to it