They certainly would go all out to woo the South away from the US/Japan. There would be an all-stops-out courtship. SK politics and public opinion is erratic enough that it may work.
Maybe we are seeing a precursor to the fall of the Communist regime in NK — like East Germany when the USSR fell and the reunification of Germany?
Maybe through their own experience, China have realized that Communism is a failure and that it is time for NK to end it.
South Korea would probably be happy with a reunification of some sort as long as the current government in NK is gone.
South Korea’s economy would certainly be able to transition the North into the modern world.