Mac Performance guide fields a question on why their isn’t a new MacPro tower.
And hopefully they have it right, that Apple is not dropping it, but waiting for Thunderbolt 3, which can go over a USB 3.1 cable, and leave it all one port.
Still doesn’t answer the need for real PCI slots and the ability to use normal video cards (especially NVIDIA cards), but it would make for a much simpler machine. If a port can be USB, Video or Thunderbolt with the same connector.