I don't think it will be possible, there will certainly be many global currencies created because there will be a lot of economic developments that may in the future use digital currencies and will use both digital currencies and fiat currencies for global currencies.
I know many of us recommend bitcoin and other cryptocurrencies but i think only dollar recurrences can rule the world. Bitcoin is slowly banned and i think in future it will banned everywhere.
Even dollar cannot rule the world. It can be accepted as a form of payment globally, but it cannot just be used as the only single currency in the whole world.
In this case, what you are trying to refer to is the idea where countries will have to discard their own currencies and adopt only one, and I want to ask you if you think US government will have enough power to be able to impose their currencies on all countries that are independent. I totally doubt that. Worst that would happen is to see USD still standing the chance to being recognized in terms of usage worldwide, but cannot be used as a single global currency.