The only chance that we can have a global currency for this world is the internet based currency as there will be no control from any country based on blockchain.
FIAT can done it as there will be an issue on whose country will be the creator of it.
When we have some as authority of the global currency then what is the purpose of making one.
I don't think it is possible to make such currency without internet and blockchain based.Just announce gold as the accepted currency everywhere which is the only option.