I think the most extreme and remotely plausible thing NK could want to do (but that would already mean taking an incredible risk) is to direct a nuclear missile

I think the most extreme and remotely plausible thing NK could want to do (but that would already mean taking an incredible risk) is to direct a nuclear missile to explode somewhere on the ocean, far from any populated place but close enough to the west cost of the US to send a clear warning about their capabilities to reach a major US city if provoked. NK is never going to actually attack the US first, it would simply mean squandering all the effort made so far in building a credible deterrence. Deterrence is not deterrent anymore when you force the enemy to respond.