Issyk Kul is bigger, but Karakul Lake is closer to China and much closer to disputed territories.

The disputed territories in the west are already domestic and foreign issues.

I highly doubt China is going to take Kabul, but if you look at a map of Afghanistan, you'll see why the Wakhan corridor shouldn't likely be a hard get. China's west needs water. That part of Afghanistan has water. The unanswerable question tho is...will Beijing ever care?