The possibility to see bitcoin become global currency all over the world is so tiny, as we can see that there are some countries does not like the existence of bitcoin. To be a global currency means all nations should make it as a legal thing but so far there are only few countries legalizes bitcoin. I dont think it will be a global currency ever.
maybe we need to wait for a few years later and when the time is come, maybe all nations will make bitcoin as a legal thing and in that time, we can use bitcoin as a global currency. bitcoin itself needs time before its legal in all nations and every government can take benefits from bitcoin itself so they can make some regulations before they accept bitcoin.