The Xeon E3 line does not offer any significantly new product lines or capabilities, besides ECC memory, which isn't that important. I think its more likely Apple would build a new product that contains a high end core i7/Xeon E3 than have a stripped down mac pro and still call it a mac pro.
Apple's desktop lineup is currently Mac mini (good) -> iMac (better) -> Mac Pro (best). Is there a good reason for this to change? There is are clear differentiators between each of these products. The mac mini is the cheapest way to get a mac, but has very compromised performance and is often neglected by Apple. The iMac is the desktop mac for everyone, with a great screen and reasonable performance, but at a premium compared to PCs. The Mac Pro is for high end performance and is the most expensive.
Right now, the high end iMac intersects the low end Mac Pro. This is not a new development, its just that both the iMac and the Mac Pro got more expensive (the Mac Pro first, followed by the 5k iMac). Your mac designs are nibbling at an idea I have had for a mac that fits between the iMac and the Mac Pro. A future mac line could drop the mac mini, make the iMac the entry level mac, add in a "Mac Pro Mini" followed by the Mac Pro. There are a few reasons I could see this happening.
The iMac seems to have heat and throttling problems especially on the high end 5k configurations. Drop the 90 W CPUs and the ~125 W GPU and use the 65 W intel CPUs that include the iris pro graphics. This makes the iMac the entry level choice, and its an all in one that is easy to setup and use. Prices range from $1000 to $2000 and the same 21" and 27" retina configurations. Savings come from dropping the discrete GPU, and the economy of scales ramping up for the retina displays. This makes the defining feature of every affordable mac the gorgeous screen.
Jumping to the Mac Pro, drop the 4 core configuration leaving 6 core xeon processors and dual GPUs. The same tube design we love (and hate) that starts around $3500 to $4000.
This leaves a gap that could be filled with a headless mac, the "Mac Pro Mini". Using the Mac Pro as a template, an even smaller mac is designed around the thermal core with a high end core i7 processor and a ~150 W GPU. This results in a compact mac that can plug into a retina external display and is still cool and quiet, because it isn't restricted to the thermal constraints of the iMac. The new AMD Fury Nano and future HBM based GPUs can play a part in this, because of how small and compact they are. This is a nice desktop that is significantly different than other PCs and laptops to justify a cost starting at $2k.
This leaves iMac (good) -> Mac Pro Mini (better) -> Mac Pro (best). However, I don't think enough has changed in Apple's eyes to justify such a switch. I think they would rather people step up from an iMac to a Mac Pro, without offering something in the middle.
I was thinking the same concept of Mac Pro Mini, but then it would be competing with all the general PC's sold everywhere with low margins. And Apple would not go there.