According to Japanese newspaper Nikkei, Apple is currently looking into OLED screens for a 2018 iPhone release.
More than just a 'stab in the dark' rumor, Nikkei claims this to be fact, alleging that Apple has begun discussions with factories on screen production, worried about possible supply shortages. OLED screens are a certain improvement over Apple's current technology, allowing for brighter, thinner, efficient and more vibrant screen.
While OLED seems like an obvious choice, this technology hasn't yet been viable for most - with issues being based around supply quantity and cost of production. With Apple looking to make the leap into OLED, it's likely that other manufacturers will follow suit, but we'll have to wait and see to be sure.