A few key assumptions
1) The dollar as reserve currency must go. Political instability in the United States has reached the point where the US can no longer be counted on to be financier of last resort.
2) The Euro must be cracked. At a minimum some nations need to be let out of the Euro. There is the possibility of a dual North-South Euro system but something needs to replace.
3) A place needs to be made for China and eventually India over the coming decades.
A potential suggestion: A global central bank, the GCB.
The as its name suggest the GCB will operate as an international central bank. However, GCB currency will be virtual and float against all existing currency. Their will be no physical GCB notes and no retail vendors will be expected (perhaps even permitted) to accept GCB notes as tender.
Instead GCB notes exist solely as a medium of account and exchange for international transactions.
The GCB will issue currency notes as well as bonds and use the proceeds to purchase sovereign debts. As a first step I might suggest a portfolio that mimics the fraction of world GDP from each nation.
Membership of the board of governors of the GCB would be comprised of a single member from each major country, with a vote in accordance to that countries percentage of global output.
For example we imagine that there are 100 votes total. The US would have one representative with approximately 30 votes. This implies that districting countries will not alter votes and of course that vote share will change as countries grow and mature.
The goal of the GCB is to create a reserve currency and a market for bonds deep enough to support global finance without being tied to any individual currency. Also, because GCB notes do not count as tender GCB policy – I suspect – will not serve as monetary policy.
Thus any nation will be free to appreciate, depreciate or float its currency against GCB notes as it sees fit.