The biggest theoretical issue in macroeconomics is “what causes unemployment”. As discussed in the last post, the classical answer, that unemployment is caused by problems in labor markets, is obviously wrong as an explanation of the simultaneous emergence of sustained high unemployment in many different countries. Unemployment is a macroeconomic problem.
The central macroeconomic policy issue, then, is “what, if anything, can macroeconomic policy do to move the economy back to full employment”. If you accept that, under current conditions of zero interest rates, there’s not much positive that can be done with monetary policy[1], and you stay within the bounds of mainstream policy debate, this question can be restated as “how effective is expansionary fiscal policy” or, in Keynesian terms, “how large is the fiscal multiplier in a depression”.