For sure, many customers have been reaming that the future iMac would sport a Retina Display, with an expected resolution of 5120x2880for the 27" model. However, according to the available sources, it would not happen. It seems that display manufacturers could not produce provide such display, with such specs and in expected marge volume (without speaking about the price).
Indeed, the price for the 15" Retina Display is so high, that even after removing optical drive, etc. from the standard MBP, this model remains more expensive, due to its display. S, imagine that a 30" REtina Display would require a surface to be manufactured, four time larger than the 15" display. So, we can expect that the price would then be at least 4 times higher, making simply not affordable/compatible to be installed in an iMac. Based, on current rumors, Apple pays 150 USD per Retina Display, so for a 30" Retina Display, it would be around 600-700 USD, if you add all other components, it would simply become impossible to have iMAc for less than 1800 USD.
Last but not least, to manage and offer graphical performance level for such 30" Retina Display, you would need an amazing GPU to only provide 2D rendering, and if you still want to have enough GPU power for 3D, then you might need dual or even more GPU. So, the bill would keep increasing.