Economics 101 says that if China's economy tanks, the price of commodities will drop. In 2004, they were using about one-half of the world's supply of cement and steel. They're importing oil in an equal amount with Japan, and they're buying gold for their central bank. Even if it doesn't "tank", but slows down notably, there would at least be a levelling off of prices.
Historically, China has never sought to acquire new lands. Its claim to Tibet is way old, and Taiwan was part of Mainland China until the Nationalists moved into it in 1949/1950. The exception might be in Siberia, but I don't see China going to war with Russia over that corner. They seem to be doing in Siberia what illegal-immigrant Mexicans are doing in southern California, only far more quietly.
The puzzle problem for China for which I have no answer is the ratio of young males to young females, and I don't have enough knowledge to predict the results of that. It doesn't look good, seems to me.
Art