I think it is very unlikely that we will see a worldwide crypto currency issued by a central bank since that will require a government so powerful that it can impose his will above the will of the other countries and while the U.S is very powerful is not that powerful.
I think cryptocurrency will not be able to master the major currencies in the future, as long as the government has a fiat then it will be difficult for cryptocurrency to be the main currency in the future
I thin that one day crypto currencies will come in in our life as something normal. Yes, today governments do not accept crypto currencies as something legal. But i am sure the day will come, cause crypto currencies become more popular this days, add to that it is the greatest place for investment. Today we can make payments without paper money, Applepay PayPal and other services that make our life easier. That all is become more useful today and popular, so I think it's just a question of the time when governments get involved in all this crypto world.