A world currency will not happen any time soon since it would deny any country the ability to wage war against another.

If you would have a world currency, nobody would be able to manipulate the currency in order to build an army, which comes at great cost. Look at what happened in Germany over the last century or more recently in the US who has de facto been in default since August 1971 when the conversion to gold was "indefinitely suspended". This has been true for all great powers, back to the Roman empire really. All these wars cost a lot of money and being able to print is the only way to finance them, for a time at least.

A world currency would mean relative peace and this unfortunately, as good as it sounds, is not what many leaders and vested interests would like to happen any time soon. One can make lots of money by wedging wars...