A single-government system or a North-South Korean alliance?

Unification under a single government
 
After the fall of East Germany, West Germany absorbed the territory of the East and formed a new system under a single government. In the case of the two Koreas, a single government would likely lead to an improvement of political rights for citizens in the Northern territory. While it would be unlikely that the movement of citizens across the country would be restricted under a long-term plan, the new system may implement initial temporary restrictions on movement.
 
Under a single-government system, it would be possible to institute government organizations in the North specifically tasked with managing regional development. Various organizations could be placed in charge of the Northern region’s development, welfare, political stability, and other areas. 
 
It would also be difficult to justify the restriction of movement of citizens beyond a limited amount of time under a single-government system. Inevitably, there will be confusion in the early stages of free movement. In parallel, average monthly wages for North Koreans would almost certainly rise. When wages stabilize, the migration of people from North to South is likely to diminish.
 
The tax and welfare systems of the North will also require a special transition period before a single nationwide system can be established. Although a great deal of financial investment will be pouring into the region, welfare needs in the short term will be significant. 
 
While there are many potential pitfalls for a single-government system, it may represent the fastest route to political, economic, and social integration of the North and South. 
 
A North-South alliance
 
An alliance between North and South Korea would be seen as a form of partnership rather than unification, but such an alliance would be an important first step towards unification. 
 
At the same time, Kim Jong Un is not likely to accept any kind of alliance, as it is an inherent threat to the regime’s hold on power. 
 
Ideally, if the Kim regime were to be replaced by a rational government which favored reform and opening, and desired a thawing of relations with neighboring countries, such an alliance may be possible. But it is highly unlikely that a rational and stable government would appear from the ashes of a sudden collapse of the Kim regime. Any hope for a new government to establish better relations with South Korea and China or to institute reform and opening is predicated on the near impossibility of a smooth initial transition to such a drastically different authority. Despite the low likelihood, the two countries could still form an alliance while maintaining their existing separate systems. 
 
Similar to the former states of the Soviet Union that established independence after its collapse, North and South Korea could also form a modern commonwealth of nations. Under such a system, there would be no overruling legislature, but instead a central decision-making organ, a council of heads of state, a council of prime ministers, and a cabinet committee. North and South Korea could maintain separate national defense and diplomatic apparatus, but establish joint committees on unification, language integration, cultural assimilation, economic cooperation, and other areas. 
 
Under a joint unification council, representatives from both countries would be able to work on addressing restrictions on movement, economic cooperation, separated families, language integration, and non-political areas including school curriculum standardization, cultural exchanges, humanitarian aid, professional skills assistance, and student exchanges. However, the leading authorities of both nations could still hold veto power over any decisions made through the unification committee. Military-related matters would also need to be handled by a top-level summit instead of by the unification council. 
 
Instituting an alliance like this would face numerous challenges, simply due to there being so many parties with vested interests looking to gain influence. Nevertheless, such an alliance would likely result in more development on the peninsula and in Northeast Asia as a whole, regardless of whether or not the two nations eventually unify.