3x Apple 12x Bionics will outperform an 18-core iMac Pro in Geekbench CPU performance. This doesn't take into consideration the differences in thermals and power management of an iPad vs. a Desktop Workstation.
I believe the new MacPro will scale from a few (2 chips, 16 cores) to a lot (8 chips, 64 cores) Apple A-series chips. These will be held in some module or node format, and if we are lucky, we will have the ability room to more modules. I believe this is where the majority of the engineering effort has gone into, solving for shared memory and interconnection.
I believe that this paves the way to iMac, MacMini and even MacBook Pro to move to arm as it will not needing to develop specific desktop/workstation class chips.
Each module/node will have 2x Thunderbolt 3 ports, which will serve as a video output and as high-speed connectivity. I do not think the new MacPro will have any PCIe slots.
I believe there will be a “system” module/node which has a disk, analog audio, network and maybe some regular USB ports.
Besides the engineering challenges, I believe the MacPro has partial delayed because of Thunderbolt. Up until very recently, it was a very intel centric technology. Today not so much.
The new Apple Display will have Face ID and utilize an Apple T-series inside the display to handle some pairing to authenticate between the MacPro and Display. I hope they take the whole front facing array from the iPhone Xs and place it in the display. :)