I predict that China (or specifically Qin Shi Huang)'s new expansion-related ability has something to do with the damming/redirecting of rivers, as a nod to the ancient Dujiangyan irrigation system, constructed during the Qin Dynasty. The Min River would previously crash down the mountains and slow down abruptly, making the surrounding area extremely prone to flooding, and rather than damming the river, the Chinese decided to redirect it to irrigate other areas. That would work perfectly with this expansion and its new features.
Perhaps China gains the ability to redirect rivers, or gain some sort of bonuses for the land fertility thing related to rivers that was shown, or some other way to turn a flood into something more positive.