That's not possible but hypothetically let's assume that such a thing will happen, now we all know that US can produce any number of currency notes when compared to other countries because of them being a super power. Now if they start adapting such policy of one currency world, who will decide on which country should have the most power to supply currencies without corruption involved?  In an economy there are various things that need to be looked at before even thinking of doing something. People will talk about  the good and bad sides of it the whole day without being tired.