If there was ever a hope for a joint EU fiscal authority and eurobonds, it's got to be through the technocratic mandate of the ECB. There is way too much national mistrust right now plus Germany is dead-set against any debt monetization scheme. However, if the fiscal authority is within the strictly-defined mandate of the ECB to manage the money supply with no spending capacity attached, I think it has a chance of gaining support. Here is how it can work.
National governments set the VAT level at let's say 20%. The ECB commits to funding the respective government at 17%. In normal times, the ECB can set the VAT Credit at 3% with no impact on the money supply in the respective country. The effective VAT is 17% and the national government receives all VAT proceeds. However, suppose that Spain is in a recession calling for loose monetary policy; however, Germany is at full capacity calling for a tighter monetary policy. The ECB could set rates to respond to German conditions, but increase the VAT credit from 3% to 10% in Spain to mitigate the impact of tighter policy there. The effective VAT in Spain is now 10%, however, the ECB covers the revenue shortfall of 7% by issuing Eurobonds. A mechanism similar to the debt ceiling could control the maximum Eurobond issuance.
The VAT credit could be even more powerful at the lower zero bound where the effectiveness of monetary policy is greatly diminished. The ECB could increase the size of the credit such that the effective VAT is negative thus creating conditions consistent with negative interest rates. Basically, central banks would be in a position to expand the money supply through monetary flows in the real economy rather than newly printed money balances that people can simply turn around and stuff in their mattresses or bank accounts. I discuss at length here why negative taxes on consumption and investments credits are much more preferable at the lower zero bound than actually charging negative interest rates or even eliminating paper money as has been suggested by some.
I strongly believe that granting short-term fiscal tools to central banks merits further consideration. Such proposition creates all sorts of possibilities for central banks including the ability to customize monetary policy by region as in the case of the ECB as well as more broadly, providing central banks with new tools to operate at the lower zero bound.