Nothing will happen if NK get lots of nuclear weapons except that we won't invade them or maybe even put sanctions on them. You have to be completely and certifiably insane with no regard for your own life to lob a nuclear missile at the US. That would be a death sentence for their entire country and they know it. As long we don't invade they won't fire missiles because Kim like all dictators wants to live and run his country.  A worse problem is if Kim gets desperate for cash and decides to sell nuclear tech to governments or even non government groups who are certifiably insane enough to sneak a nuke into the US and set it off.