Marco makes a good case how Apple will update the Macs to Retina. Assuming that we can live with a reduction in resolution similar to what we got with the MacBooks, where we went from 128 dpi to 2×110 dpi, we would need 108/128x110x2 or 186 dpi. I see the following options for new screens:
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3840 x 2160, 24", 184 dpi
Requires 11.124 GBit/s. This will be the only iMac size offered from now on.
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4480×2520, 28", 184dpi
Requires 15.141 GBit/s. This I hope will be the update for the Thunderbolt Display. It would have at least 2 GBit/s free to support Ethernet and USB, and the bandwidth would be just low enough that later you could daisy chain 2 of them with display port 1.3.