Put international pressure on China to prevent it from propping the regime up any longer due to it's fear of the humanitarian crisis at it's doors.
China is the one that's keeping the Kim family fat, wealthy and in power. There is no answer without Chinese assurance, and that's as unlikely as denuclearization. The US and the other SEA powers (including South Korea) cannot allow North Korea to do as it pleases while also disregarding all efforts at amenable diplomacy. One side cannot be the one that constantly gives while the other refuses to negotiate in good faith or even adhere to any agreements for a length of time that is determined.
The sad truth of the matter is there is no good answer to solve the issue. The transition is likely to be bloody, messy, and the aftermath primed to be the great humanitarian crisis of the 21st century. 25M+ people are going to be left a country with no functional infrastructure, logistical capability stuck in the 1960's, a complete lack of capable internal governance thanks to having literally no political education and a lack of food and arable farmland.
The best hope for any real change is the vain dream of internal reform. But as long as the DPRK has Chinese assurance as to it's survival, that's unlikely at best.