Preface: I don’t know what the new iPhone looks like. I don’t even know whether there’s only one new iPhone. The only thing I know is that there existed, months ago, “N94” prototype iPhones that looked like the iPhone 4, but which contained the iPad 2’s next-gen A5 CPU. That proves nothing, however, as I believe Apple was just as likely to build iPhone 4-lookalike prototypes whether or not they ever intended to bring them to market. Testing new internal components in disguised exteriors is an obvious strategy for a secretive company.
So what follows is just good old-fashioned speculation and design thinking.