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.
This discussion has some of the politics involved in it as well. No matter how much Bitcoin flourish in the US and the European countries, US will never let it damage its national currencys value because that is of prime importance to the US and US earns a lot with this USD Dominance thing. So Bitcoin can have a chance to become one global currency and this is one sweet dream however I do not think it is a realistic one.