As to the idea of collapsing the iMac and iMac Pro lines into one using the iMac Pro case and cooling, it would be feasible and possible.
The W-Series Xeons are just the X-Series Core i9's with ECC RAM support so Apple could conceivably just smash the lines together - I expect ECC RAM is not really critical to most iMac Pro users, but it is what comes with Xeons. Intel did launch the 10th Generation (Cascade Lake) X-Series last fall at 10, 12, 14 and 18 cores and all are at 165W TDP and priced at less than half of what the W-Series were.
Apple is clearly seeing significant drops in NAND storage prices which is why they have been doubling their base SSD capacities. So I could see them dropping Fusion Drives in favor of 512GB SSD as the base level of storage. The iMac Pro case (including cooling) also likely costs more than the iMac, so I could see Apple raising the base price by at least $100 to cover all this. Maybe raise it $200 and bump base RAM from 8GB to 16GB. I mean people are going to erupt over the loss of upgradeable RAM so might as well raise the price, too.
And when you consider adding 16GB of RAM and a 512GB SSD to an existing base model 27" iMac is $500, paying $200 for it is a deal.
So where does this leave the iMac Pro? Perhaps as an option package instead of a separate model?
MCK says a 27" miniLED display is coming so what if it becomes a BTO upgrade option like the X-Series (12-18 core) CPUs? Same with 10GB Ethernet? And workstation-class GPUs?
So you have the "base" 27" iMac with the 10500 Core i5 CPU, 16GB of RAM and a 512GB SSD for $1999 compared to the current 8500 Core i5, 8GB of RAM and a 1TB Fusion Drive for $1799. I like to think that reasonable people would find that a reasonable deal.
From there you can add RAM up to 256GB, storage up to 4/8TB, faster Ethernet, workstation-class GPUs and miniLED display.