Given that Trump knows little about nuclear weapons and less about Korean history, and has no apparent inclination to learn anything between now and June 12, it is impossible to believe that he will leave Singapore with a good deal with Kim. That leaves two alternatives: a bad deal; or none at all.
What would that mean?
A bad deal would take one of two forms: either a vague concept of phased arms and sanctions reduction, with the details to be filled in later; or a detailed version of the same thing. The first could easily morph, over time, into no deal, given Bolton’s opposition to a diplomatic solution and North Korea’s record of ultimately walking away from phased agreements. In either event, the world would lose its appetite for enforcing sanctions, and Kim will wind up regaining his economic and diplomatic contacts without losing all of his nuclear program.
No deal could mean either an agreement just to meet again, or an acrimonious split. Either way, enforcement of sanctions will be weakened, and Trump will be faced with the stark choice of either containment or war.
In short, unless you’re rooting for Trump to win a Nobel Prize, don’t get your hopes too high. On the other hand, it would be kind of fun to watch Trump and the GOP twist themselves into a pretzel defending an agreement which provides fewer protections than the Iran deal that they just trashed.