I will go on record to say that it will be called the iPhone 5N.
The N is going to stand for NFC. It will make passbook more useful and Apple will act as if they invented NFC though no doubt their approach will be powerful enough to actually make NFC relevant for everyday people.
Aside from that, expect maybe slightly better camera optics, processor/speed bump, battery life bump, possibly ability to do Voice + Data on Verizon/Sprint.
It will look exactly like the iPhone 5 though, so no exterior changes. Perhaps some tweaks in the process to make it more durable or get rid of scuffgate or something but it will not be a radically new design.