I think every country already has its own currency. But for now bitcoin remains the only digital currency used globally, if for a single currency I think it is not yet time.
Such a beautiful question. It reflected unity for once. Well let me remind you of the time when Great Britain had set out to conquer the world. They had been ruling over a major part of the earth. Every single man under the British Empire used the same pound used in Great Britain.
More than half population of the world was using the British Pound. If we look at the technological innovation in the world right now, this is very possible that in the future one coin will rule the world and everyone will use it.