It would be so awe-inspiringly fantastic if he actually went and did it, whatever it would entail (executive action, joint action with congress, or just sheer bluff . . . it wouldn't matter to me) for a number of reasons.
1. Both political parties and the media overlords in the U.S. (as well as intelligentsia) have been more or less "bitch slapped" (my wife's words not mine) by this guy getting elected. Kicking the U.N. out of the U.S. and withdrawing from it as a member would have a similar effect on more-or-less the entire "rest of the planet"
I can seriously think of nothing else that would be quite so awesome . . .
2. The lamentations and hand wringing of the various ideological factions who tend to take issue with Trumps Revolution of modern political order would go into full orbital retard mod, and they might never achieve re-entry. In effect, their minds would be blown, and the capacity to engage in the fake news peddling, shit-stirring and passive-aggressive diplomacy games would be more or less ended by beheading.
Lets consider for a moment, what would be likely to transpire if Trump were to broach the topic of "U.S. withdrawing from the U.N. and revoking the U.N.s permission to operate on U.S. soil . . .
Of course the first reaction would be wild hyperbole and disaster prophecies on the part of the Young Turks types of the Internet. This would be followed by anger, seething anger, showing of asses, blind rage, insane conspiracy theories, full fledged psychopathic rage fog, implosion of the medulla oblongota . . .
The whole point really of the U.N. is to give the police states and the pipsqueak states a "legitimate" framework to attempt to undermine the powers and superpowers. If the U.S. is no longer a member, I doubt the U.K. would see much point in staying either. France would likely follow. Italy, Spain, the low countries, Canada, Australia, would likely follow not too much slower. Germany might take a while, they are still overcompensating for that "Nazi boogeyman" image so they feel they need to be really careful about such things I guess . . . Once those initial wave of powerful democratic nations were effectively pulled out, many others would follow: Japan, South Korea, most of Eastern Europe (Finland would probably stay in for some perverse reason . . . its like a rape victim being in the same "community welfare" organization where its rapist is a member of the "Security council!").
Within a year, the only remaining members of the U.N. would be Russia, China,_possibly_ India, and the various developing nations that stand to benefit from the U.N. (and which in many cases have their own regional or political organizations that already replicate much of the putatative benefit and function of the U.N., the Union of African States for example . . . Most of Sub-Saharan Africa could drop out of the U.N., resort to the UAS for whatever "benefits" they lost as a result of dropping out, and be just fine (except for the chance to parade around repellent diplomatic overtures that turn the bigger powers against one another, else turn the small powers against segments of the big ones.
I doubt any wars that are not already well on there way to initiating would start, and I doubt that any lives would be lost, fortunes ruined or real harm done to any human beings . . . well, that is not entirely true. The ruling regimes in some nations which make a career of "playing the U.N." as well as the "technocrat" class of bureacrats who comprise the U.N. would most definitely lose out big time. But that is like saying: disbanding this mafia will definitely cause the criminals in the mafia to lose out big time.
Win win for the U.S., the EU, most of the developed world, in truth, most of the developing world too. I can see no down sides to it.
Hopefully Trump makes it so. However . . . he does something that extreme, the Secret Service are going to be more busy, because there WILL be some assassination attempts, most notably from Russia and/or China, who are probably fairly proficient at that sort of thing.
The assassination attempts would most likely come from the democrats and the GOPe.