that sounds less like a rumour and more like something you just made up.
-would iPhones shift from 16:9 aspect to 4:3 aspect as well?
-if a single UI design fits all form factors, that goes against the entire argument apple made for separate phone and tablet versions of apps.
if apple does make a new screen size, my bet is they are going to abandon the rigid pixel doubling (which has already sort of happened by making the OS vertically flexible with the 5/5s). responsive design is the best approach going forwards, or they will forever be painting themselves into corners.