"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
AirPods and AirPower: Everything We Know
iPhone 5 Rumors Spawn Mockups
In particular, the talk of a 4" screen and rumors of an "edge to edge" screen resulted in this mockup by mjay2k which shows that a 4" screen could fit on an existing iPhone 4 sized device by extending the screen to the left/right edges. Similarly a smaller iPhone could use the existing 3.5 screen in a similar fashion.
Another take on the "edge to edge" screen, submitted by a reader. This image tries to look like a "spy" shot, but we believe is just another creative mockup.
Finally, this old design mockup pre-dates the iPhone 4, but shows another take on a large screen format with a reduced prominence of the home button.