The happy end would be a unification of north and south.
Unlikely. Neither the Chinese nor the Russians would be extremely happy with a close ally of the States on their doorstep instead of the current heavily fortified buffer zone.
The Chinese would be much more concerned than the Russians. The Russo-Korean border is only a few kilometers long, and I don't think that the NATO will install bases anyway near that narrow strip of land. On the other hand, the Sino-Korean border runs through three strategically important provinces of China.