Takeshi Fujimaki, a banker turned opposition lawmaker, said the Bank of Japan will achieve its 2 percent inflation target much earlier than expected. And it scares him.

The yen will slide toward a more "appropriate" level at ¥180 to ¥200 per dollar over the long term, from about ¥123 now, as the BOJ sticks to bond-buying stimulus and the Federal Reserve embarks on successive rate increases, said Fujimaki, a member of Osaka Ishin no Kai, which is led by outspoken Osaka Mayor Toru Hashimoto.

While that will help the monetary authority reach its price goal before the fiscal half ending March 2017, it will add to the risks of "fiscal collapse," he said.