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
It can actually be main currency for the entire world to be used on the internet but in cannot be a country specific main currency. In this regards, you can expect bitcoin to be the major currency which is being accepted in the entire world for all digital transactions, this way I do foresee it will be beating paypal hands down.