I hope i will live enough to see sovereign debt being given only with a physical collateral: a piece of national territory. Otherwise, no way currency union, or any other supranational union except those backed by force (like USA) can survive. If EU leaders had balls, they will prevent what's happening with Greece now, but nobody wants it to do in their electoral cycle.
Debt must come with strict, enforceable and unavoidable means to collect it, otherwise both sides - lender and recipient - if they are sovereign nations - are making troubles for themselves and others.