It's a good thing that the Kim dynasty is over however if Un is dead, his sister will most likely take over. Her views towards South Korea, the US, and the western world is just as bad as her family before her. History has shown that powerful female leaders will do crazy and stupid things to get attention and the respect they want from the rest of the world and given the current situation with North Korea and the tensions going on between us and South Korea, these next few months will certainly be interesting. Basically she will be less-cool version of Azula if she is the one in power. This however will be the best possible outcome of what could happen.
Worst case scenario we get is that there's some sort of coup or a civil war in North Korea to the point where the nuclear weapons they have are vulnerable and could potentially be used. China would have to get involved, South Korea too, and us as well and Korean War 2.0 happens.