Let's see, Kim Il-Sung conveniently dies in 1955 in a plane crash and his son is too young to succeed him so the Kim Dynasty is nipped in the bud.
Instead Pak Chong-ok, leader of the Soviet Koreans faction, comes out on top and ushers in an era of deStalinization and thereby follows Khrushchev's line. Later North Korea's leadership starts reforming in the 70s and 80s a la Deng Xiaoping as their centrally planned economy starts showing flaws.
Defence spending ITTL is high, but not like OTL. The army of NK doesn't exceed 500.000 men in total, it's main battle tanks are the T-72, the T-80 and two dozen T-90s purchased from Russia in the 1990s, and then are several hundreds T-55s and T-62s in mothballed; the airforce mostly has MiG-29s and Sukhoi Su-25s, but there are also still MiG-21s, MiG-23s and some Su-22s in motball facilities. NK has no nuclear weapons program (it does have nuclear energy for peaceful purposes), but does posses chemical and biological weapons that can be put on IRBMs.
South Korea sees Park Chung-hee die in the 1960s and a different military dictator takes over whose economic policy thoroughly fucks up the South Korean economy. By the 1980s, South Korea has turned into a banana republic reliant on US aid to keep up military superiority...
By 2011, the authoritarian, socialist North Korea has achieved GDP growth rates up of to 5-6% annually with a 21.500 dollar GDP per capita (about the same as Portugal) while South Korea is on a similar level as Russia with a 10.000 dollar GDP per capita, high levels of corruption, poverty, crime and an aging infrastructure.
North Korea, for all intents and purposes, is now the best of the two Koreas. It has a mining sector, steel industry, a booming consumer electronics industry, a highstanding car industry and the country gets lots of tourists, among them many hikers and mountain climbers who want to climb North Korea's mountain ranges.