iPhone 5, iPhone 6, etc.
Whenever I ask or am asked "what can Apple possibly add to the iPhone", there is usually silence. I don't have any great ideas and haven't heard very many (kickstand! barometer!). But, to see what sticks to the wall, here's what I've got:
1) barometer/thermometer. why not? they are simple and potentially useful.
2) infrared. probably not because it feels like a fast disappearing technology but there are a bunch of potential apps that would come out in days, if not hours, if this hardware were built in. If apps are the lifeblood and distinguishing feature of the iOS ecosystem, adding a $1 transmitter seems like a good idea.
3) back touch panel. This is tricky because it creates a huge device fork but there are many instances, particularly for games, where touch on the front screen is a problem (impedes the view, etc.). Cases would also be a big hassle. But there are lots of times I feel that it would be nice to touch the back to indicate an input. It also makes one-handed operation easier. And it would truly make the device "touch".
4) pressure sensitivity. Again, this really takes touch to another level. And creates a whole set of intuitive gestures. And a big device fork.
5) feedback. Creating resistance, a sense of softness, etc. could be super powerful. Cases would be a big problem again. As would device fork.