I can think of a few ways it could happen. It could spread from an already democratic section, like Hong Kong, or Taiwan. It could be a populist revolt. Or, it could be granted by the existing Chinese government, which would work wonders on its retention of power. Let me assume that, and discuss the existence of a democratic China, one year after the transition.
Taiwan's last objections to rejoining the mainland are gone, and the Pan-Blue movement of Taiwan overwhelms their Pan-Green opponents to vote to rejoin the mainland. And there was much rejoicing, although not in the Pan Green parties.
The Chinese nation now has 1,361,732,740 people, living on 9,677,012 km
On election day, the Chinese voter has some 14 political parties to chose from. I predict a tight race primarily between the CCP (having gained popularity from allowing democracy) and KMT, with a very marginal victory by the CCP, which must then establish a coalition to govern effectively. The results will largely depend on how the system is structured, with some systems working better than others. I think a European parliamentary system could work, or the Israeli system, but the US style "winner take all" would not work. (China's political opinions are diverse enough that a "winner take all" system would self destruct within 3 elections.)
Internationally, I predict that absorbing Taiwan would mellow China towards Japan slightly, but not by much. It would still treat Japan as an arch-rival, and glare at it suspiciously with every chance it could get. I predict it would dump North Korea like a hot coal, and enjoy warm friendship with South Korea. A cool, mild like of America would also ensue.
I also anticipate that the Chinese guy will, at his convenience, point out numerous things that I turn out to be wrong about.