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. 
I think it reached that level it was not easy thing you know ? bitcoin is currently not so well known and very difficult for bitcoin to be able to get the legalization of a one country ? because the government thought bitcoin is a threat to them and their currency ?