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?
The experience of the European Union shows that it is very difficult. And considering the different state of the economies of the countries of the whole world and the different mentality of the countries, this is basically impossible.