I based on plausible leaks I've been tracking, I consider the next mMP to be as follows:
Form factor: desktop, mini tower, about 18" tall, vertical airflow, CPU/RAM/PSU in the rear half, 2 GPU PCIE in vertical at the frontal "lobe" triple width, room for a common optional radiator on top, GPU has to include internal display port interface, legacy dp/HDMI interface's accessible thru an optional adapter Linking it to the back end.
Frontal lobe in space gray, rear lobe in space Black.
Rounded corners as the Mac mini, open top for natural convection, dual ~20cm fans on top.
CPU: no real clues, assuming apple and AMD has working own thunderbolt 3 compatible host Chip, we could see an Epyc - TR3 mMP with PCIE 4 , else apple will be stick with Intel Xeon-W and PCIE3
TDP ~ 900-1000W but "quiet" configurations may need barely need 250W
RAM, 6-8 slots ECC DDR4
Storage: ridiculous expensive apple PCIE but x8 interface faster than anything on Earth (.. )
GPUs: at launch AMD Vegas only, later nVidia as bto or as diy upgrade, AMD Navi may not come to the Mac this year
Ports: 6x TB3 4x-6x USB 3-whatever, 1-3 HDMI 2.x or DP1.4
No real cues on a 3rd PCIE slot for non GPU s peripherals.
Apple may launch it's own TPU for training/inference no cues if pcie or TB3
Base mMP cost the same as current tcMP 6 core, but starting with 8 cores and a single passive cooled GPU (Rx vega 48 likely)
Apple professional display to be usb-c/tb3 6400x2880 pixel, curved
A new pro desktop keyboard is possible including the touch bar and slim mechanical switches.
No chance for 2p sata m.2 neither arm switch.