China can't go north due to Mongolia and especially Russia, unless
they want to find out what the phrase "Stirring up a hornet's nest"
really means. Of course I could see them taking control of North
Korea once that insane asylum finally comes apart. The residents
of that place would probably find Chinese rule a relief by comparison
to what they have now.

Their western territory is mountainous, desertious, and basically not
very conducive to comfortable living.

To the east is the Pacific Ocean. Well, I suppose they could set up
off-shore habitats. After all, undersea cities were another thing I was
promised would happen in the future as a kid (Sealab 2020).

That leaves the south. They already have Tibet. And then there
is Pakistan and India. India might be a crazy gamble, but if things
become really unstable in Pakistan, I can think of a few nations that
might look the other way if China went in to keep the Islamic
extremists from using their nukes, and then they decide to stay for
a while to maintain the peace, of course.

EP - China and Russia may not hate each other as much as they
used to, but don't kid yourself or anyone else that they are really
friends and allies now. You're just hoping China will implement CAPS,
even at the cost of looking the other way at their dictatorship and
human rights record.

An asteroid strike is far less likely than a powerful nation trying to
dominate Asia and more.