As coalition military spending dwindles and aid projects come to a close, Kabul last year launched tax audits of major contractors to the U.S. military in a bid to bolster the country's revenue.

WSJ: Kabul's Tax Levies Raise Flags From U.S. Watchdog

One promising idea, from Germany's Council of Economic Experts, is to mutualise all euro-zone debt above 60% of each country's GDP, and to set aside a tranche of tax revenue to pay it off over the next 25 years.

ECONOMIST: The euro zone

Second, it's hard to find a country that would prefer not to have oil and gas because the revenue stream is so uncertain.

BBC: Fact check: The boys from the black stuff