Yeah the only existing currencies I could see taking over are the euro or yuan, but Europe is too politically diverse and China is too politically hostile.
The only existing "currency" I could see is some kind of opening up and expansion of the SDR[1] (China has pushed for this as an alternative way to replace the USD as a reserve currency)
Europe's political diversity is a benefit here though - the EU Council as a governing body, provided it got to the point where it promised not to change anything, would easily look like a stable administrator. Business is more then happy to ignore all sorts of legal inconsistency if the trade off is predictable year over year behavior.
The only existing "currency" I could see is some kind of opening up and expansion of the SDR[1] (China has pushed for this as an alternative way to replace the USD as a reserve currency)
[1] https://en.wikipedia.org/wiki/Special_drawing_rights