Based on rumours so far, the big tentpole feature of the iPhone 7 will be the new material it's made of which will eliminate the lines on the back.
People who have the ridiculous idea that Liquid Metal is radio transparent, are leaning towards it being the new material but it's actually not, (it's not even a "material" per se).
The most likely bet is Zirconia crystal like on the back of the Apple Watch, assuming they have solved the brittleness problem at least to a degree. Second most likely is a sandwich or other combination of Liquid Metal and Zirconia.
For instance, an iPhone designed like the 4s as a "sandwich," but replacing the glass on either side with Zirconia and the metal part with Liquid Metal would be virtually indestructible and would actually bounce when you dropped it.
Apple also has published many patent in regards Liquid Metal inserts or "windows" in Zirconia crystal and Zirconia "windows" in Liquid Metal chassis'. One of Liquid metal's most important and unique characteristics is that it's a poured material that does not shrink appreciably when cooled, so it could easily be poured into a mould, that contained Zirconia inserts and it would form around them seamlessly.
Still far more likely that we will see a full Zirconia chassis though.