How would this work? If this currency is backed by banks and countries, then it looks like a centralized coin, and it would just inherit all the problems that we already have with multiple currencies, in every single country or union. I mean, this would be like dollar or euro, but in a world scale. I don't think this would solve anything, and I don't even think this will ever be possible.
I don't see how a coin could be shared by the US and Russia, or Iran, or China, etc. These countries simply wouldn't work well together. They can tolerate each other, but working together is a complete different thing.
I do think that a decentralized currency could exist worldwide, and I'm of course talking about bitcoin here. But it needs to be decentralized and not backed by anyone. Just a currency that gets it's value on a free market, ruled by supply and demand.
While Bitcoin will continue to exist as a decentralized currency, I believe that in the future, there will be a single centralized currency which will be backed by any country in the world. Having a single ledger which is shared among countries is a huge plus towards preventing criminal activity at a global level. Also, it gives governments the advantage of tracking every people in the world in the way they buy/sell stuff, transfer funds, and even their personal identifiable information.
A centralized system which has blockchain technology as its backbone, and AI, will allow a robust system that will handle the likes of every government worldwide. While it sounds creepy, it could possibly happen in the future, eliminating the need to create a separate digital fiat currency for every region. By having a single digital currency, it could be strong and unstoppable as any country in the world supports its chain.
On the other hand, Bitcoin and other decentralized cryptocurrencies will co-exist with the centralized currency, they will be used at a minimum unlike the latter. At least, that's how I see it. Just my thoughts