TOKYO — Among enthusiasts of a certain age, there was a notable flicker of excitement back in March when Nissan announced that the Datsun brand was making a welcome and long overdue comeback. Starting in the late 1960s, Datsun hit a strong chord in the U.S. market with cool, well-conceived, good-value entries such as the Datsun 510 sedan and sexy 240Z coupe, effectively the Japanese BMWs of their day. But Nissan has very different plans in mind in reviving Datsun for the modern era. The target is high-growth emerging markets such as Russia, Indonesia and India. A new family of...