[throwaway920102]

pretty much all economists and commentators state that the replacement will be a basket of currencies not a single currency, where each countries' basket will be weighted by the typical trade volume they have with other countries (more trade with Country X, then hold more of Country X's currency) plus some factor for stability/volatility/currency risk etc.

[decimalenough]

This, incidentally, is exactly how the value of the Singapore dollar is managed.