Next-generation iPhones likely to focus on internal improvements.
Apple Designer on Why Curved Glass and Extruded Aluminum iPhone Designs Were Rejected
Most notably, Satzger reveals that Apple very much wanted to use a curved glass design for the iPhone, but cost considerations and technical hurdles forced Apple to change directions.
The technology in shaping the glass, the cost relative to shaping the glass at the time, and some of the design features of this specific shape were not liked. [...]
The technology at the time had a lot to do with it. The qualities of the glass at the time had a lot to do with it. These are models -- I'm trying to remember a time frame -- that were before gorilla glass and before a lot of the other factors.
"0355" iPhone prototype with curved glass on front and back
Satzger also addressed Apple's ideas for an extruded aluminum design similar to that of the iPod mini, noting that the design was rejected for both comfort and technical reasons.
My recollection of it was that to get the extruded aluminum design that was applied to the iPod to work for the iPhone, there were too many added features to allow it to be comfortable and to work properly. [...]
If you put an iPod up to your ear, the sharp edges, because of the processes, aren't comfortable, and you can't get antennas to work properly in a fully enclosed metal jacket. So each one of those things needed to apply other features that started.
iPhone with silver iPod mini-like extruded aluminum design (Source: The Verge)