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'm sure cryptocurrency never become the main currency countries. It always is and will remain an alternative currency. People do not like something new. And do not trust