I really believe that the world can't deal with a global currency all together. There are politics all around the world that changes everyday and the relations with each country changes in between them, one day two countries are allies and another day they are putting economic pressure on each other, unfortunately the "wars" of these days are not done by weapons but it is done via economic regulations, now it is great that people are not dying anymore because of the wars that is an improvement however there are a lot of poor people because the greedy wants more money for themselves and punish the poor for not doing what they ordered them to do.
This causes whole nations to crumble because of one bad move by politicians of that country. Global currency would mean countries that want to punish other countries would have to end up using the same currency and that wouldn't work that well. Look at what happened with euro, germany is doing great but greece is not doing so well (historically never did) and while germans are increasing the value of euro the greeks are dropping the value and that hurts the german public for no reason at all, why would anyone want to get that to whole globe when europes doesn't work properly.
You do have a point there, mate. However, I believe that at some point in the future, governments worldwide would come up to an agreement as they work together to solve the world's problems. When universal laws are devised, then everything would behave like a single country no matter the region or area you're located at. This would take some time, as the world is slowly shifting to the digital sector. When globalization takes effect, and technology advances to a certain degree, then expect for a "One World Currency" to become a reality in the future.
Right now, Blockchain technology is still in its infancy. At the same time, Internet access is not available to every region of the world. As such, I believe that some time is needed for a complete transition to everything digital. Then, all your personal, financial, and medical information would be stored in a global blockchain which proves their authenticity. The same way, food supplies would be tracked via the use of blockchain technology for complete transparency. And last by not least, a world currency would make use of this technology to give governments the control they need among their citizens. Here, they would be able to track every transaction made on the global ledger and restrict or censor access to someone's money in case they see anything suspicious.
The New World Order would effectively be achieved this way, as everything would be trackable via Blockchain technology. There will be no privacy, as surveillance would become massive by then, and AI takes controls of decisions without the need for human intervention. Therefore, Blockchain technology would be ideal for not just a "One World Currency" to take effect, but also for nearly everything we've been accustomed to in our lives such as our personal, financial, and medical information. Just my opinion