3:2
4" iPhone and a bigger iPod Touch:
Apple can't lower the pixel density on its next iPhone (and iPod Touch) under 300 ppi, being their defined retina-class display pixel density for pocket devices' screens. They also won't make a phone much more bigger, especially in width. Though, the iPhone screen is never wide enough.
While the Tablet category probably allows any aspect ratio, the Pocket category of devices has more constraints.
I don't (want to) believe in the 16:9 iPhone rumors; I prefer the 3:2 screen aspect ratio for pocketable devices. It is said iOS could be able to accept height increase in resolution, as it is already asked for devs to support notifications bar events. Ok, but once held in landscape mode, the added pixels would be in width; what would then happen to apps? Anyway, i just like better the actual 3:2 aspect ratio.
For a usable screen a minimum size is imposed for obvious reasons (eyes capacity, average finger size / minimum touch area size),
If both portrait and landscape mode are targeted in usage, with constraints of hand morphology and to fit a pocket, you can't make it too large, but as you still need surface to display information, one end up with 3:2 – 8:5 as maybe the right aspect ratio range for pocket-class devices.
And for example, 16:9 (and over) aspect ratios are too elongated for thumb-usage only, out of a reasonable size of screens, and almost prevent from landscape mode usage if the screen is too small (well, of course, you can always find a way to still use it in landscape, but you need enough pixels, so increase size, and that's probably part of the reason of higher screen dimensions in the 16:9 Android world, coupled with often smaller pixel density).
And a 4:3 aspect ratio imply rapidly losing too much in height to keep it not too wide.
So let's consider they will stay with a 3:2 aspect ratio, having proved to be working fine from the start, and ensuring a better compatibility with existing apps.
How could Apple deal with the increase in size of the iPhone's screen while still support its apps catalog? Pixel doubling again would be overkill and technically, about doubling pixel density of screens is probably not feasible at low cost, or at all. But how about simply tripling the original iPhone resolution (i.e. 1.5x the retina iPhone screen)?
Once screens attain a pixel density in the retina range, the blur caused by a 1.5x increase become less perceptible, as Apple showed us with the retina MacBook Pro used at the non-native 1920x1200 resolution.
LG demonstrated
recently its ability to produce 440 ppi screens, achieving to attain the limits of eyes capacity to distinguish pixels for people with perfect vision.
At 1440x960 (1.5x the iPhone retina display resolution), the 440 ppi screen reach 3.93" in diagonal, with an increase of less than 6 mm in width compared to the actual iPhone screen size (and the whole device would simply go back to the width of the 3GS), and a screen surface increase of more than 23%.
And then, you also have the iPod Touch to take in consideration: at this resolution, a screen at a pixel density between 440 and 326 ppi would allow to make a pocketable but bigger iPod (5.3" at 326 ppi), with still app compatibility (usage wouldn't suffer much from the expansion compared to apps running on the iPhone 4"). [more on this in the picture description]
And ultimately, once 480x320 support dropped (say 2 years from now, considering the 3GS will probably not be sold anymore after next iPhone refresh this autumn), the resolution would have to be seen as 2x 720x480 (and to support the then old 4/4S/Touch resolution, could be rendered at 2/3 until their turn to be unsupported)
Any adjustment in the iPhone or the iPod Touch size after that, if necessary, would be possible in the future by 'simply' adjusting pixel density of screens and leaving unchanged the resolution, with no real visual difference/adaptation for users/devs.