Actually there already is an international group which could be the replacement.
If we had the cojones we could transfer UN responsibility to G7. It makes a lot of sense from the standpoint that these are the countries who end up paying for almost everything the UN does.
All of the third world countries who do nothing but endlessly debate the faults of the US and the wealthy countries could be given observer status on the condition that they STFU and accept the handouts we give them on our terms instead of demanding them on their terms.
Plus, we could yank the diplomatic immunity of all the observers and make them learn to park their cars in legal spots--just like those of us who pay for those cars.
The G7 is an interesting suggestion!