Could you see a future wherein the entire world shares one global currency, be it fiat or crypto? If I think it in a scifi kind of way, it would most probably be some digital currency that could all just use. It doesn't even have to be in this century. Maybe the next?
Of course. It could already be a possibility right now given enough adoption into the bitcoin network. There's already the means to do it in the form of bitcoin, all that is required is an increase in adoption worldwide to the point where bitcoin can be used worldwide without having to exchange any currency.
Obviously, that would be a pretty utopian view by our standards and probably won't happen any time soon.
More likely than not I think that if there was to be a global currency, which is a likely possibility in the future at some stage, it will still be issued by some sort of central entity, potentially a world central bank of some sort.
The other more realistic possibility is that bitcoin will not replace fiat currencies outright around the world, but rather be a globally accepted
alternative to fiat. It's still a global currency, and it holds value wherever you go without having to exchange forex. But I don't think that in that bitcoin as a parallel global currency scenario you can say that it's the "single" global currency around the world.
But hey, anything could happen. Bitcoin could become the dominant currency for international business over the internet, as some sort of niche. We don't know at this point other than the fact that BTC is capable of being just that.