As the game progresses, trade routes can become a good source of extra food, hammers, and commerce. When the enemy starts pillaging all your roads, farms, and mines, airports with the ability to resume trade can significantly help to keep the city up and running. This ability doesn't warrant a new building, though, and tacking it onto an airport makes sense. Also, if you build a new city on another continent and it doesn't have access to the coast, then buying an airport can make the city grow very fast (due to extra food and hammers from lots of trade routes.)
Then, the World Wonder could just give all cities an airport.
The art for Silk Road is from Wikipedia, so I guess it's fine if others want to borrow it.
PlotLSystem.XML seems complicated. I hope it works, but it's too many variables and attributes to traverse. Strangely, I can't find anything on watermills or riversidemakesvalid that has to do with orienting the structure. (I've searched in XML, Python and SDK.)
On to more ideas, it could be fun to fight over trade routes, or at least hurt your opponents'. What if there are new invisible units called robbers and pirates that steal from your opponents' trade routes, if they stand outside or stay on a ship outside an enemy city. On the flip side, there are new patrols and coast guard units that can see these invisible units and fight them.
Also, what if you could accumulate food and hammers like you do with gold. You can put extra food and hammers into a national treasury when you tell a city to stop growth or stop production. Use the food to help another city grow faster, and use the hammers to finish any production. These could be enabled through tech or civics. Perhaps you can trade them with other civs, just like gold or gold per turn. Not only that, convert from one type to another: gold for food or hammers, food for gold or hammers, and hammers for gold or food.