Apple might introduce a 17" rMBP with high-end features like two SSD slots and a 32GB DRAM option, but it would be similarly thin like the 13" and 15" rMBPs so no Ethernet, no user-changable DRAM, no HDD, no optical brick. They won't bring back Firewire or serial or parallel ports or floppy discs either.
If Apple do introduce a 17" rMBP, it would probably have a speedy discrete GPU and that would be the end of discrete GPUs for the 15" MBP.
The reason why Apple dropped the 17" cMBP is that in 2012 17" Retina displays had very low yields and so were prohibitively expensive. Now yields are up and prices are down. In 2012, Apple would have had to choose an inexpensive CPU to keep the price of a 17" rMBP with 4GB of DRAM and 128GB SSD down to $4000. That would not have sold well. Apple can now make a $3000 17" rMBP with a speedy CPU, 16GB of DRAM and 256GB SSD. I don't know whether or not Apple will do so, but it's possible.