The work that remains is not political, but economic. If the Euro is to work, there will need to be considerable attention paid to the countries that are currently close to being in line economically, but are not quite there yet. At the core of the union are Germany and France, the two largest economies on the continent. Belgium, Luxembourg, the Netherlands, Austria, and Finland are smaller nations that have managed to get their economies pretty much in sync with Germany and France. Finally there are four nations, Italy, Spain, Portugal and Ireland, that for differing reasons fail to line up well enough for long term success.
Economic alignment is an essential element in any unified currency.
To show this, one can look at the simple equation reflecting the relationship between spot and forward exchange rates where currencies trade freely. This equation states that the forward rate equals the spot rate adjusted by the interest rate differential. Any time interest rates in two countries are different, the spot and forward rates will also be different. In the case of the U.S. and Japan today, low Japanese interest rates mandate that the spot rate is above the forward rate, i.e. it takes more yen to buy a dollar today than in the forward market. For currency union to hold, spot and forward rates must be equal, and interest rates must be aligned.