This obviously isn't an iPhone discussion, but I've postulated that Apple could double the resolution of the iPhone 5, stretch it onto a 5 inch screen and have a "Super Retina" display of sorts. 2272 x 1280 pixels is still less than the Retina iPad (2.9 million versus 3.1 million) so such a device could use an A6X and be very speedy and relatively efficient.
How does that sound for a Super iPhone (iPhone X or Pro or whatever they would call it). But I would imagine the price would be $849 or more for the "base" model.
They need the IGZO screen to move to a full HD (1920x1080) bigger iphone. IGZO will allow more pixels with very low power consumption. In the furure I expect Apple to continu to carry the 4" screen and add a bigger model. They may also make a less expensive (but not cheap) iphone for emerging markets.
The thing I would also love them to do is to start building iOS phones for others, like facebook, yahoo, amazon, ... This would be a nice way to expand the part of ecosystem (like apps) and still turn a profit out of it.