It can't be possible for the world to become one nation, the world is too big for everyone to be doing things in common. I think the world is never meant to be one, the way which the world is right now with different nations is the reason why the world is developing through the differences in currency, economy, and resources.
yes, it's possible by only decentralizing the system otherwise it's not possible cause only Bitcoin or ETH or another nice coin can make it.