The iPhone doesn't need to get any smaller, and the display won't need any more increases in resolution. That's practically it's only noticeable physical feature. Anything they add to it at this point will be hard to notice unless it adds substantial new functionality.

They can always improve on speed, camera resolution, and storage; they could add LTE and NFC, but I can't think of much else that would even need hardware support that they couldn't add in software.