I've suggested Port-au-Prince before.
Close enough to keep an eye on, out of our hair, and would add some economic diversity to the Caribbean.
As if Haiti doesn't have enough problems...8^)
Seriously, the 18 acres of land that the UN sits on is worth, what, $200-400 million. They could buy some good land outside of Paris, France, by the airport and build a new facility for so much less than building a temporary NYC structure, then rebuilding the UN over 5-6 years. Start building a permanent new facility in Europe today and move permanently in 18 months.