I agree, my guess is the iMac is updated this year. An i7-7700K with an AMD RX 480 is good enough performance that it will kill off the Mac Pro as currently designed.It would be better to allow the ipad pro to pair with the imac pro for use as a drawing pad, versus the touchscreen.
TB3 ports out the ying yang, xeon, ecc ram and a pro gpu - all doable on iMac and rMBP.
Pretty safe bet the Mac Pro is dead soon. Yearly iMac update and mobile workstations make more sense. Kills two products; MP and Display.
To keep the Mac Pro relevant Apple would need to go to a dual socket + dual high end GPU design for differentiation. I don't see Apple actually doing that and most likely the Mac Pro will be killed and they will be touting how amazingly powerful the iMac is. They may use the "Pro" moniker, but my guess is from the sounds of things they will be marketing VR capability instead. The Mac Pro would actually sell pretty well if you could get it with dual NVIDIA Pascals.