Just like East and West Germany eventually became one so do the two Koreas desire unification. All it needs is for USA to stand back and let it happen. The fact that North Korea is aggressively working on 
development of nuclear weapons is caused by USA policy in the region. Americans through their fear of communism could not let go of Korea. Perhaps now is a good time. If they can tolerate a communist China they should be able to accept the free expression of the 2 Koreas whatever form that would take.
Russia was big enough to let go of Germany.