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?
One global currency can only go when there are no significant contradictions in the political and economic sphere of human activity in the world between states. In my opinion, this will not happen in the near future. However, even if such a currency ever appears, it definitely will not be a cryptocurrency. Cryptocurrency has great price volatility and thats the problem. Significant international trade transactions in cryptocurrency are impossible due to the fact that one of the parties will constantly lose them.