Things that won't happen, but which would be cool:
1) biometrics. could integrate with keychain access and thus eliminate the need to constantly have to enter passwords with the on-screen keyboard (a particular pain in the arse given that good passwords mix letters, numbers, and punctuation, and the built-in keyboard error corrector can't correct them for you).
2) camera integrated into the lcd, so that teleconferencing not only works, but you don't appear to be looking in the wrong direction. Apple has a patent application on this tech., and with cmos sensors it should be doable someday.
3) solar? maybe some day, but I doubt it would provide enough battery boost to be helpful.
4) SD (or similar) slot. Won't happen, but it sure would be nice.
5) replace the front button with a button that can also act as a rocker. in combination with the accelerometer and camera, would make iphone one of the best portable game platforms out there. Would also facilitate one-handed operation of productivity apps, and wouldn't detract from the aesthetic if done properly (that is, it can't jut out).
There won't be an EZ-pass transponder. Too many incompatible variations nation- (and world-) wide, and in fact, most people don't have any use for such a thing. However, building an ISO-compliant contactless card interface in would enable all sorts of fun things.