What fiscal rules for the euro zone?

In November 2019, French president Emmanuel Macron called into question the rule limiting fiscal deficits to 3% of GDP. But if this rule, for which there is no theoretical justification, is abandoned, it would need to be replaced by another fiscal rule, as many euro-zone countries (Germany in particular) demand one. What could a new fiscal rule entail? Like for local authorities, public debt could be limited to financing public investment, and debt financing of current fiscal deficits could be prohibited; But the rule should go further: public investments that give rise to a fiscal deficit should only be permitted when they are sufficiently efficient that they generate enough additional GDP and tax revenues to cover the cost of the debt ; Crowding-out effects must also be considered: if there is more debt-financed public investment in the euro zone, could it lead to less private investment? This is unlikely at present: If, in the event savings are scarce, public investment is limited to investments that are sufficiently efficient , generat ing higher returns than private sector investment; Since the euro zone has a significant savings surplus, which i s currently len t to the rest of the world.