In “normal” times, a transition of leadership in Pyongyang would be a cause for tension, but probably not something that would be disastrous. With a designated or clear successor, a world not preoccupied with a pandemic, and an actual thinking human being as leader of the USA, one who hasn’t done his best to gut our expertise and capability in pretty much every aspect of our government, including the intelligence and military areas, a new leader in Pyongyang would be one of those periods of uncertainty and tension, sort of like when the USSR had a transition, though maybe a bit more wobbly.
Right now, though, all bets are off. I still think the most likely scenario, if Kim Jong Un is actually unable to continue as leader (for whatever reason), is some form of cabal or troika or whatever that will rule collectively for a period until a consensus pick emerges (read, kills all of his/her rivals). This group will likely not want to stir up too much trouble, as they’ll have their hands full, though they will probably bluster and rattle sabers a bit to prove their bona fides.
Now, a rational US leadership would understand that, and play along enough to make sure that the rattling didn’t get too dangerous but also that no one on our side of the DMZ overreacted. But, um, we don’t have a rational leadership, or any likelihood that the people calling the shots in Washington have the experience, temperament, or knowledge to ride this out in the right way. Two ways, at least, it could go pear-shaped. One, the US takes the bizarro path blazed by Trump already and signals weakness and support for the new regime, emboldening them to do goodness knows what, miscalculating perhaps that the US would actually acquiesce in some adventurism they might want to get into. Or two, the US panics and in an effort to look strong/“I’m in charge”/fuck COVID-19, here’s where I can triumph!," overreacts and provokes the North Koreans into doing exactly what we have hoped and prayed they wouldn’t do, for fifty plus years.
Luckily, one weird side-effect of our current inept and ignorant government is that it’s entirely possible that our actions will be totally ineffectual, and Chinak, the ROK, Japan and others can keep the lid on.