I doubt Apple will go over 324 DPI anytime soon. Doing so would require devs to maintain yet another separate sets of bitmaps, and modify their touch target sizes.
A res change from 1136 x 640 to 1280 x 720, while seemingly minor would require a modification of their app assets/bitmaps. Also the touch targets wouldn't need to be modified with intelligent scaling. Although I still think Apple would prefer pixel doubling as they've done on 4 of their different products in the past.