What if all nation use only a one currency or one global currency?, What will happen to our world?, What is the effect of every nation's or every country economy?, It can make people's unite each other?Any opinion.
If that were ever to be the case, then hopefully this 'one world currency' is one that is governed by a system of decentralised issuance - or else we'd all be subject to one mega, monolithic, global, central bank.
Power will be concentrated within this one organisation, and they'll have the ability to do alot of crafty things to further empower themselves at the expense of the rest of the world's population.
However, if this one world currency was (or similar to) Bitcoin, then this may also bring about a fair share of turmoil. Bitcoin is a deflationary currency, which benefits it's earliest adopters - putting them in a position of power over 99% of it's users. Alot of interesting things can be done when you possess more than 20% of a digital currency's entire supply.
In short, a "one world currency" would be disastrous for the progression of economics in general - and will cause alot more harm than you'd naturally expect.
I personally think we are fast on track to a system of "niche currencies." Currencies based on specific markets, uses and functionalities. This is a departure from our current system of 'geo-political currencies.'