As others have said, basically it's a problem of control. Normal currencies, in relatively normal times, are semi-managed by central banks. They use tools like

As others have said, basically it's a problem of control. Normal currencies, in relatively normal times, are semi-managed by central banks. They use tools like reserve requirements and interest rates to influence the rate at which new currency flows into the general economy. It's possible for one bank to trade in the market in a way to peg one currency against another, as China did for some years. And it was costly, because they were doing it while trying to maintain a large trade surplus, so large numbers of dollars were coming in that needed to be neutralized somewhere. It's why they hold so much US debt. Real currencies get in trouble--severe deflation or inflation--when circumstances (the gold standard, external debts financed in another country's currency) make it hard to respond to economic conditions. This becomes the question: can the system be designed to respond dynamically to demand in a way that mimics the actions of a central bank, without there actually being a central bank? If the goal was to make it so that it was reasonably constant in terms of purchasing power, there's probably a number of metrics one could choose to measure that, but how to achieve it, that's above my pay grade.