See this Reference Link.
Difficult to say. The fiat currencies we have today, have built-in fault lines within them, when it comes to issuance and usage. The US Dollar certainly is the preferred reserve currency of choice at the moment, but if history has taught us anything, it is that nothing lasts forever. Especially empires.
I think a lot of countries will eventually start to experiment with the cryptocurrency world. Whether they do it on the existing cryptocurrencies like Bitcoin, Ethereum, etc. or their own coins or even a combination of the two, remains to be seen.
Acceptance is the main factor at play here. Can a country park enough funds in Bitcoin and then also be able to trade them with their partners, without that wealth being manipulated in price? Not to mention, is there enough coinage to be assigned to countries for their reserves? all without the volatility? I’m skeptical, but hopefully that some new variant, may do exactly that. Become the dominant reserve currency of the world, like e-gold and cannot be subjected to price or forking manipulations.