Guys I have been thinking a bit about the possibilities about next Mac Pro, and came to conclusion that everything what Apple will come up with at 2999$ as starting config will be simply milking their customers.
If Apple wants their hardware be competitive, I think, has to do this:
Quad core Xeon E5 - 1620v4 3.5 GHz/3.8 GHz,
16 GB of RAM DDR4 ECC
512 GB SSD
Dual Radeon Pro DX300 based on Ellesmere Pro with 2048 GCN cores, each and 4 GB of VRAM.
Price? 1999$.
They could then combine the "need" from the community for xMac.
It would not cut their margins, because the GPU can be found for under 200$, so AMD would sell it to Apple for under 150$ for each GPU, CPU is 294$, RAM can be sold to OEM's for as low as 50-60$ for 16 GB kit, 512 SSD 120-150$(Apple gets their own memory controller anyway, and other things related to SSD hardware).
Then they can always bring 6, 8, 14, 18 core configs, as BTO options, up to 256 GB RAM, 2 TB SSD, and another 2 tiers of GPU hardware.