That would basically almost be similar to what we currently have with fiat currency but instead it would be backed in cryptocurrency. I personally do not think it would be a good idea because one of the main reasons for cryptocurrency is for their to be a decentralized economy and if each country has their own coin then those governments could regulate it to their liking and we would still be back to having separated currency.
Yes,crypto currency have to be centralized if it was created and backed up by something then it is same as the fiat money system and it won't bring any changes apart from the current system.So it is not a good idea though for us because we will not get any additional benefit from using those centralized currency.And there are some countries already started to think about the crypto currency for their own but still no country created I think.