At present, each country has its own currency. Can the future focus on a single common currency?
I don't think so. Currency value, reflects the value, of a community. If there is growth, currency value, will be higher, otherwise the value will be lower. A single common currency, cannot reflect the financial value, created by different countries, in the world. If you look at cryptocurrency, there are a lot of different coins, that are used for different reasons. The single currency idea, exists, because, we are used on a single government, regulated currency. That's why we have financial problems around the world, because it does not work. Cryptocurrencies, were invented to solve this. Let's see if they will, in the long run.