Japan could sure use a new source of energy, like North Korean coal. They would probably be leery of building up North Korean fishing operations though. The Japanese could probably profitably off-shore some manufacturing to North Korea, and develop some tourism facilities for Japanese.
South Korean tourism also has large potential, as do guest workers or day workers from the North in the South, depending on how they manage the movement of people.
The North Korean ex-patriot population in Japan (some third generation, yet still holding Korean citizenship) could play a unique role, as they have largely retained their Korean language ability. They are associated to some degree with Pachinko gambling parlors and the Yakuza underworld, although they are in many walks of life.
The big Korean Chaebol will likely expand aggressively into the North, if the rules are set right, and China could likely lay in some cheap roads and rail in 2-3 years. Although North Korea lacks a lot of infrastructure, their rail network is relatively a strong suit, having been built up during the Japanese occupation before WWII.
The Chinese might also benefit from trans-shipment of their products through North/South Korea, to avoid tariffs.
Russia should be able to get in on the growth as they share border along the coast, but probably won’t, unless there is some oil or gas to produce.
Big Australian, Canadian and American mining majors should be interested in North Korea as well, and they are able to raise large amounts of capital investment from outside the region.
The only unknown is whether the Kim family is willing to give its absolute power to revive the country.
Despite North Korea’s nominal Communist trappings, is really been a heriditary absolute monarchy ruled by the same family for its entire existence.