Probably an upgrade of the Discrete Graphics on the GPU front, ie replace the 650 and 680 with 700 series equivalent
It will be interesting to see if keep on nvidia or go amd with the graphics card.
As only just changed the case, then I wouldn't have thought that will be making any changes there.
An upgrade to 802.11ac wifi following the use on the MBA and the new Airport Extreme and Time Capsules.
As there are already BTO on the iMac for Flash Storage, then I take it you mean a lowering of the Price, or the ability to add a second Flash Drive.
Only way I way I see for 2 drives in a BTO is when you buy a Fusion Drive where have 1 Flash and 1 HDD connected as 1 volume in Fusion.
With the MBA, rMBP, and the nMP/iTube all only presenting a single space for disk then I suspect that likely to remain as as single volume offering so just the 1 Flash Disk option. Only the mini server has an option for buying with two volumes now. You want more then one volume/disk then you go external.