World currency in long time period can be Bitcoin, especially, if many not so developed countries, like India, China etc. will implement it. For this step all countries need Internet network developing. In medium time period of course USD play its role as world currency. Do not forget that many people like paper fiat too, so all paper fiat money is more or less strong. 
yeah I think that the internet would be a very flourishing future later possibly all financial transactions can be accessed and used online. and you could see today that fewer and fewer people use paper money right ? people prefer to use a credit card, or debit card for shopping at supermarket
well that depends. if we're talking about how widely used these currencies are right now, it's still going to be the dollar, and I think it would be so still in the next couple of years. although digital currency is slowly gaining more popularity, when you say world currency, I think the dollar would still the basis for now.