When you talk about "2x" and "3x" mode, are you referring to the way an iPhone app scales up on the iPad? Because that isn't relevant here at all. (Unless I've misunderstood something...?)
The base iPhone app size is 480x320 points, the 5S size 568x320 points. The aforementioned 1472x828 resolution is actually 734x414, and as you can see none of these iPhone resolutions fit into it twice at all.
It has nothing to do with being able to zoom. It has to do with physical size. Basically a 'point' should be the same physical size on all devices. This is very important for a touch based operating system. Doesn't matter what the display resolution is - you want to know that a button is always going to be 0.5" wide. If you ask for a 50pt wide button - you will get the same sized object on all iphones/ipods/ipadminis. However on the retina displays you will need to provide an image with twice the resolution - hence the name 2x. So apple use a point system that i meant for physical size - and then you must provide a image of either 1x resolution or 2x resolution.
It seems unlikely that they will add anything except 3x or 4x. For example I can't see them making a 2.38x mode. That basically means separate images will be needed for iphone6 than all other devices and the images will be only slightly bigger resolution. Most people are assuming 3x is the minimum jump, some say only 4x makes sense.
Until they're updated, you'll have the app surrounded by black borders equal to the difference between the app's supported size (480x320, 568x320, etc.) and the size of the screen.
If the DPI is higher than 326, then you can't really do the black border trick. For example if the DPI of the iphone 6 is 440, then old apps will be 25% smaller - all the buttons will be 25% smaller in width and be harder to press. Apple could probably scale up the apps - but they would look terrible.
There's really only two ways I think they can go personally:
With 736x414 on both devices which is easier for developers as there's only one new "workspace", but the 5.5 drops down to ~306 PPI.
If the 4.7" had 736x414 in 2x mode, it would be 360dpi. This means buttons etc. would be 10% smaller physically than on an iphone 5s. It seems backwards to make a bigger display - only to make objects smaller. The bigger display should be to display more content and possible make objects bigger.
Or with 736x414 (or similar like you mentioned) on the 4.7 inch, and a somewhat higher resolution for the 5.5 inch to keep it around ~360 PPI, but it's at the cost of having to support two different workspaces.
EDIT: Also forgot to mention 2208x1242, but then PPI hits 450 and you're looking at a 25% reduction in physical object size compared to the 4.7 inch, which shouldn't be too bad, however you've got the added issue of battery life with such a high pixel density display (needlessly high.)
If you ignore the leaks (which probably aren't credible) - then there is no reason to suggest anything other than 326dpi for both 4.7" and 5.5". This makes everything so simple. All phones have the same size for objects. Both new iphone 6's will display old apps at their exact original size and resolution. Devs will only have 2x to worry about, and simply have two new canvas sizes.
For the 4.7" I would say 326dpi is almost a dead certainty. Apple gain nothing with a higher DPI display except lost profits.
As an engineer, if I sat down and was told to make a 4.7ish screen and 5.5ish screen, I would keep 326dpi, and then to make it easiest for developers - I would make the canvas jump in linear intervals from 5s to 4.7" to 5.5". So whatever the resolution increase is from 5s to 4.7", increase it again by the same count for the 5.5". Since the 4.7" is 0.7" bigger than the 5s, i would make the larger one 0.7" bigger than the 5s. Not 0.8" bigger. The result would be 5.4" with 1728x864.
Very simple elegant solution that is happy for everyone. Devs have two new resolutions, but at least the canvas jumps are linear - and therefore dynamic scaling is far easier to do. 2x mode is kept for everything. All dpi's are the same and physical sizes remain the same. Old apps will display exactly as they do on 5s/4s. Its wins at everything *except* being 5.5". Remember that the iphone 4 was not 3.5", it was 3.58". Likewise the iphone 5s is not exactly 4" - its a metric display that is neither 4" nor 326dpi, its just very close to those numbers.
