For now there is no global reserve currency candidate other than the US dollar, the Euro and the Japanese Yen. The Chinese renminbi isn't mature yet but it can earn the status of a reserve currency in the future if China finally opens its economy (respecting free market principles) and allows its currency to float freely with no intervention from the central government. Considering the depth of the Chinese problems though, and the expected internal political turmoils, their currency is unlikely to serve as a global currency anytime in the next decade and beyond, unless they really tackle issues and turn things around.
Today, 70% of all the dollars in the world are traded outside US's borders and about 62% of all the global central bank foreign exchange reserves are in dollars. The euro holds about 20% of foreign reserves, it could exceed that figure if Europe sorts out their internal political issues and demonstrates financial stability for at least half a decade and the fact that they aren't stable economically and politically makes the dollar de facto global currency.
The Japanese Yen hasn't showed any promise yet, the Japanese economy has been stagnating for a long time and continues to lose significance as its people are aging and disappearing, so the economy is expected to shrink. You need a large enough of an economy to serve as a sufficient collateral to qualify a currency to act like a global reserve currency, this way it is less volatile and well suited for trading commodities. Gold or any other thing cannot be used because there isn't enough of it in circulation plus it's not practical, realistically speaking, the dollar is the only thing that can guarantee relative economic stability in the world for many years to come.
