OK, there it is.A 1080p-retina 24'' iMac.. I think it's reasonable to expect a retina version in 24'' this autumn then, as an addition to the regular iMac line.
I think it is very reasonable to expect to see a 4K retina 24" iMac appear. I suggested the same in another thread a few days ago. The screens are available, and considering a 15" retina MacBook Pro can drive 5 megapixels with integrated graphics, a dedicated graphics card should to be able to drive 8 megapixels.
Even the 15 megapixels of a retina version of the 27" screen don't seem beyond possibility with that comparison. Although with the ever increasing iMac screen sizes, I wouldn't be surprised to see that in 30" format.