Apple may bring back the metal casing for a 2018 iPhone
https://www.cnet.com/news/2018-lcd-iphone-metal-back-report/
Kuo predicts that next year's iPhone lineup could look something like this.
The report goes on to say that Apple could release three new iPhones next year: the 6.1-inch LCD iPhone, a 5.8-inch OLED iPhone and a 6.3-inch OLED iPhone. Of the three possible iPhones, the report only mentions that the 6.1-inch iPhone could get the metal backing.