Since I plan on using an external display anyway, would be nice to have all the upgrade options as the 27"...
But the iMac needs an update anyway and honestly 4K at 21.5" is crazy! 40" is recommended for 1:1 ratio for 4K and 43-45" for 5K.
I also want to see Thunderbolt 3 which uses the USB Type-C connection to be used on the iMac simply because this would allow for external GPUs "which thunderbolt 2 can do but thunderbolt 3 brings pci-e 3.0 and HDMI 2.0! I already have a Titan-X in my gaming rig, would be epic to use on my work machine 2!
I'd still want to see thunderbolt 2 connections with thunderbolt 3 being added, so I should see 2x thunderbolt 3, 2x thunderbolt 2, 2x usb 3.1, audio in/out "optical, and analog", microphone, and power in.
I also want to see DDR4 in all macs! DDR4 was first publically released with X99 last August and was released to the consumer line "current skylake processors", since the iMac mainly used the consumer processors we should be seeing Skylake chips inside the next update and apple used to update the iMacs not long after the consumer chips was released or even before!