There is a lot to be said for someone who is accustomed to operating with “F—k you” money in the bank.
The coup-de-grace would be that Trump kicks the UN out of the US and Trump corporation buys the property and puts up a huge brand spanking new Trump Tower.
From what I recall reading over the years, the UN buildings are in a sorry state structurally. They've asked for tons of money over the years to fix them. I would not at all be surprised if they violate countless national, state, and local statutes. They would almost certainly need to be razed.
And that's where I'd stop. Leave the demolition debris where it is, wall it off, and forbid any sort of development plans for the site. Let it stand in ruins as a permanent visual representation of the effectiveness of the U.N.