Well if not the Euro ...?

What about back to the Yen?
Nah

Japan's debt problem: Sleepwalking towards disaster | The Economist:

"Over the past two decades the country has stumbled in and out of deflation, slipped down the global league tables on many social indicators and amassed the largest gross public debt-to-GDP ratio in the world (a whopping 190%). Yet government-bond yields have remained stubbornly low and living standards, by and large, are high. Visit the country and you will see no outward sign of crisis. Politicians and policymakers have bickered and schemed, but have mostly chosen to leave things as they are.

That cannot go on much longer. The figures are getting worse. Japan urgently needs radical policies to tackle the problems, and new leaders to implement them."

More here :

Japan's debt-ridden economy: Crisis in slow motion | The Economist:

Japan's debt-ridden economy
Crisis in slow motion
Japan’s government will eventually have a disaster on its hands if it fails to tackle the deep-seated problems of debt and deflation"