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?
Why do we need a single and uniform currency? During the old days it was difficult to exchange one currency to another. That is no longer the case. So I don't think that we need that nowadays.