Supposedly Ghengis Khan had to tell his generals that they could not turn the entire northern plain region of China into horse and sheep pastures.
What if he had given them their head to attempt this?
I throw the idea (Which does not seem to have been covered by this forum.) open...
It's come up a few times. At any rate, it supposedly took around 300 years for the population of (what is today) Iran to return to the population level it had before the Mongols came. It wasn't that the Mongols killed that many people. Rather, the main impact was that the extensive aquaduct system, built up since the Persian Empire over a thousands years before, was allowed to fall into disrepair. This drastically lowered the amount of usable farmland, and thus the carrying capacity of the region.
The same thing would've happened in north China. Aquaducts would be allowed to fall apart, canals would be filled in to allow the easy transit of horses and livestock. The loss of productive farmland would have repercussions further south in China, and not just in the immediate region. Assuimg the Chinese eventually reconquer this area, as in OTL, it would take generations for the region to recover its lost productivity, and for the population to return. I personally doubt we would see much in the way of permanent territorial changes, but I think the population of this China might be lower compared to in OTL.
Who knows what changes that would have down the road?