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. 
Yup I can confirm I'm one of those people that likes paper money
I'll be the first to admit that it's not efficient (in the sense that it's not practical to carry large sums around) but digital money or even plastic don't come close to the feel of a semi-thick wad of bills 
Plus I love the smell of new bills, plastic and digital money are so boring in that regard. Also paper money is as close as you'll ever get to true anonymity. That doesn't mean that I'm against digital money, I just wouldn't mind having paper money around for a little longer.