Visually, somewhat like the current 5th generation iPod Touch, but bigger (and smaller).
From an industrial design point of view, it can easily be argued that the 5th gen iPod Touch is a better hardware design than both the iPhone 5s and 5c.
The French blog Nowhereelse.fr and designer Martin Hajek have put this idea to pictures. The design essentially looks like an iPad mini or iPad Air scaled down to iPhone size, which happens to look almost exactly like a current generation iPod Touch.
This hypothetical new design would work with multiple screen sizes. Imagine an iPod touch but sized for both a 4.7 and 5.7 inch screen…the larger would almost be like a “mini” iPad mini, but its a phone. many colors, and metal, not plastic.
The 5th gen iPod Touch design is so good, it is near “timeless” and could go on for many years (except for the camera “bump”, and lose the strap). Advancement in shrinking component size could make this style work for even a super mini with a 3-4 inch screen, if not smaller.
This device, if done as described above, could and would replace the entire iPod line as well, except maybe the iPod Shuffle, the smallest iPod. Think of the streamlining in production.