The problem is that there's not really another government debt-based currency that's:
1. running an economy large enough to support the world as a reserve currency (rules out most of the world)
2. has a government that other countries feel safe with (rules out China)
3. has current dominance in most of the world as a currency (rules out the EU, which is trying to edge into this).
The problem is that, in spite of the weirdness, US dollars are still popular with the unwashed.