So what diplomacy would be "good" with the DPRK? I doubt it's that simple when you're dealing with a goverment as single-minded as the DPRK and its "cult of personality".
I don't know. I wish I had all the answers all the time, but this time I don't. The best I can do is vague ideas and observations. Such as:
(1) One big problem with diplomacy vis a vis DPRK has been inconsistency. There's been pressure, harsh words, détente, bribery, saber-rattling, incidents, and resolutions, but I honestly can't discern any clear, consistent, strategic thinking behind it. That's done a lot of damage to credibility -- promises haven't been kept, and threats haven't been followed through.
(2) The DPRK has responded to these actions in somewhat predictable ways. In particular, they like to use the nuclear program as a bargaining chip -- they do nasty shit and then promise to stop doing it if they get X, Y, and Z. And, in fact, they *have* stopped doing it -- temporarily -- if they've gotten X, Y, and Z. OTOH they don't respond well to threats and pressure -- instead, they up the ante.
Again, I don't have any ready answers, but I think that if somebody took a good, hard look at exactly what has been done over the years and exactly how the DPRK responded to it, it might be possible to come up with a coherent strategy that could keep the threat contained. What kind of pressure works, what doesn't; what kinds of carrots are attractive, what are not; where do the real red lines go. That sort of thing.