Perhaps the form factor should be made thinner on any new iMac, but I'm not convinced screen improvement to a retina display is required. If Apple could just address reflectivity/glare with the current screens, it would be a significant improvement.
Then they just need to upgrade to new processors, USB 3, ditch the HDD for all SSD, ditch the ODD, make some further improvements in the speakers and iSight camera and I'd be all over getting one for the home office use. The current BTO with SSD already is one of, if not THE, fastest Macs out there (not sure about comparing to the new MBP's). With their volume purchases on SSD's, they should be able to lower prices on a base 21" iMac with 128gb SSD down to below $1000 and offer the 256gb SSD version for $1199 and a 512gb version for $1499.