Presumptions:
-form factor shall not be increased in any dimension, as per apple's ergonomic ethic
-screen must be bigger, as per current industry trend which is about 5 inches.
-pixel depth must be higher, as per current industry trend which is 1080x1920
-apple always pushes the envelope when it introduces something novel
-apple is very concerned with screen resolution fragmentation
-iOS products are always symmetrical on two axes
-advent of Touch ID adds complication to imminent elimination of home button
Conclusions:
Apple will increase the viewable area within the device as much as possible while preserving the 16x9 ratio. While this will be less than 5 inches, it will prevent the need for standard and plus-size models, in turn preventing fragmentation. The viewable area will not be increased so much that the overall width of the device is increased, or that there is no longer space for a home button and an equal counter-space on the reverse end. A display resolution increase to 1280x2272 (quadruple) would give that slight edge over industry standard and restore 'bestness', while the increase in overall screen size would bring pixel-depth demands into the realm of feasibility. Also, a clean quadrupling would allow legacy apps to display without dithering (a move from playbooks of original iPhone, iPad and Mac retina displays). An alternative scenario is they go the 720p route to preserve user experience, the result of which would be the current 326dpi with letter-boxing on legacy apps not unlike the transition from the 3.5" screen to the 4". In the first scenario, due to greatly increased hardware demands, I would not expect a thinner device—the space saved by igzo panels would certainly be added to enclosure volume, not shaved from form factor. If they go with scenario 2, I would expect the thinness barrier to be broken yet again. Mind you I don't think big screens = innovation, but the street and media seem to, and it is now Apple's turn to benefit.
-Kleo