I think that when an eventual military-led coup happens against the Kim regime, the new military dictatorship will probably be a lot more amenable to negotiating release of sanctions in return for regime changes. This is the best-case scenario IMO.
That would lead to a lot of bloodshed in the process of the coup though.
In the chaos he might even let loose on Seoul or Japan, whether or not he thinks the US is responsible for said coup.
IMO the ideal situation is the US does a strike on N. Korea's leadership and all known military assets simultaneously.
The US is capable of this right now and the ONLY reason we haven't done so yet is because China has a treaty with them and is obligated to save face by defending them if the US does a first preemptive strike.
Thus, the US has to wait for N Korea to strike first, and they never will. They will continue to develop ICBM tech and biological weapons and pose a far greater world threat as we progress into the future.