After years of success, the smartphone sales growth formula has hit something of a stumbling block: new features like facial recognition haven't energized demand to the degree they have in the past, while a greater focus on apps has some consumers deprioritizing device upgrades. But there are still bright spots for sales growth ahead, including some that will leverage the traditional feature-based formula. Goldman Sachs Research’s Rod Hall delves into two: augmented reality and emerging market demand.