I doubt that the imac needs a redesign next year, probably will just get a processor bump to broadwell and various spec improvements.
The next major change to imac is to make it into an external display (which I bet won't be happening until Skylake), and then perhaps to increase the screen size even if they do it with a similar overall size by putting a 30" in the same case.
Skylake is next year - late next year, but next year. It will launch in the same 2nd half of the year timeframe that you'd expect the 5k iMac revision. Intel has been terrible at keeping to their CPU schedule in the last couple years but Intel recently publicly pledged that Skylake will be on time.
As tempting as the shiny 5k Retina is, Skylake makes it worthwhile to wait if you can. Not for the CPU but the new chipset that will go with it that will have the new USB 3.1 standard that doubles transfer speed, increased power capacity so desktop hard drives can be powered through the cable and also carry Displayport video and audio signals like Thunderbolt. I would regret not having USB 3.1 if I bought the current 5k Retina iMac.
Last edited: