There's a rumor going around that Apple might be considering going to @4x -- a "Super Retina" for a future iPhone display. Apple went to @2x, from 320x480 at 163ppi to 640x960 at 326ppi for the iPhone 4 Retina display, and lengthened it to 640x1136 at the same 326ppi for the iPhone 5's 4-inch widescreen display. These days, however, Android phones are shipping with 1080x1920 (1080p) displays at well over 400ppi.
So, to leap ahead again, and to keep things just as easy on the apps as the last big density increase, in some theoretical universe Apple could just double again to @4X, right?