Getting the US out of the UN--indeed starting a "new" UN (composed of Western style democracies)--would be a fitting legacy for Bush. The UN has become the Dorian Gray of modern diplomacy. Outwardly maintaining a benign facade that hides the corrupt, rotting, maggot-eaten corpse within. Unfortunately, I doubt this will happen.
This is the only realistic solution. Never actually "kill" the UN, just let it (in Newt's words) "wither and die on the vine." Establish a new organization (a la NATO) of members with free and just governence (I hope we qualify) and defer more and more to it for decisions that affect the world.