Judging by the responses on this thread, I'd suggest to Apple to separate, but not divide, the iMac line, just like the MacBook with MBA & MBPro Retina.
iMac Lite: focus on thin, I mean super thin, form factor, SSD only, maybe touch screen (you know for the kids), good processing power (maybe iOS-like system?)
iMac Regular: current iMac, fusion hybrid SSD+HDD, upgradeability: difficult, better processing power
iMac Pro/Max: thicker enclosure for upgradeable HDD, upgradeability: easiest, external latch (like the RAM compartment in current model), SSD blade soldered to carry OS and critical files, high end DAC for musicians, best processing power, no touch screen, digital speakers (OK last item not possible).