The global currency is the one that facilitates cross-border trade, including investment and international debt markets. Global central banks hold foreign exchange currencies to protect against significant fluctuations in exchange rates, as well as in exchange rates. I think we are a long way from when Bitcoin will be able to do that.
If you look at bitcoin it is probably the best currency to replace any dominating currency for international trade and the only negative aspect you can tell is the high volatility and you cannot expect a currency to have really huge volatility and the traders will not trust anything that changes the valuation in a short period. If we overcome all those negatives then bitcoin could be a good alternative.
No doubt that today dollar is a global currency and it rules the economy of many countries as well as today
BTC is far from becoming a dominant currency that can replace dollar. Think that can happen only if dollar will hyper-inflate and the world central bank can do nothing with this. In such case
BTC can help because it's limited so it can’t inflate. Nevertheless I can hardly imagine how all that will work worldwide so as there is too little amount of
BTC in comparison to seven billions of people.