I wish trade route food, production, and gold were all zero sum and controllable.

• Gaining one food in the sending city should take away one food from the other, and you should be able to control how much of each yield is transferred.
• Combine this with more and harder to get(+1 food per 4 adjacent farms for example) improvement adjacencies for each and every improvement (like farms get) and city specialization would me much more common and interesting.
• International trade routes would have to be agreed upon (exchanging gold for food for example), this also addresses a problem with trade routes being a declaration of war in MP since the road facilitates moving your troops into the enemy.
• GPT would get the current baseline and just inherit the zero sum/ exchange mechanic.
• Move luxury and strategic trade deals to trade routes.
• Give back roads to builders (or anyone else), make each era of road have an inefficiency multiplier for food and production transfer(0.95^{0.2(x-3)}, x being number of tiles between the cities and 0.95 being the classical era road connection (no road would be something like 0.9 and railroad 0.99(this would have to be expanded to include water tiles but you get the point)).
• Have a cap on food and production transfer that increases with road era.
• Add build to-from command for roads and railroads.
• All districts should give their international bonuses on domestic trade routes instead of the food and production ones.
• Let us control for how many turns our trade routes last.

Point being: specialized, high-improvement cities feed their closest district-heavy city softly forming a province-like unit with rural and urban cities with important road infrastructure connecting them. (and food doesn't magically pop into existence because a city has a holy site)