Many news agencies, when North Korea attacked South Korea late this year, jumped to many old conclusions. The most common one that I’ve seen has been about reunification and more specifically that China will not like it simply because they would align with the United States. However, this simply misses a complex historical issue in the region. If you understand your history of the Asia/Pacific then you’d know that China once dominated the region, and so did Japan. Korea often being left to be conquered and pillaged making Korea extremely hostile to its neighbors.
I think it is safe to assume that Korea will be unified, the only question is when. At the current moment, the biggest issue is not who a unified Korea would ally with but rather who it would not ally with. The explanation of this is simple, as said before the Koreans dislike their neighbors (both China and Japan). Add in the fact that a unified Korea would overnight (with the industrial might of South Korea and military might of North Korea) become a vastly powerful superpower able to rival China. The other issue with this is that they would not just be a superpower, but a nuclear superpower. Therefore, as I’m sure you can conclude that this is not just an issue of alliances for China or Japan, but the creation of a nuclear superpower on their doorstep that hates them.
What will happen then? Well it is safe to assume that all three states in the region (the US, China, and Japan) will be engaging in satisficing for quite some time. Simply because the current system (even with North Korea becoming actively more aggressive) keeps the balance of power in the region stable.