Going to Apple silicon & Apple GPUs; the only real need for full-size PCIe slots would be the audio guys...
But I would think TB4 could handle a LOT of that need by then, either from faster "breakout boxes" or straight-up PCIe expansion chassis; Apple could do an "Applified" PCIe expansion chassis that bridged several TB4 ports together for increased bandwidth...?
So yeah, back to the "expansion by cable" method, tied & true in the finest of Trashcan ways...! But hey, is that not also the same model as the entire laptop & iMac line-ups have; expansion by cable...?!?
Actually, my idea of a new Mac Cube (see below) DOES have expansion...
It's just, well, proprietary expansion...! Yay...! Daughtercards...!
So the MPX-C (Compact) is not really like the "additional slot dealio" of the current Mac Pro, it is an Apple proprietary super duper high speed interconnect, because you can only get daughtercards from Apple (for now)...! ;^p
Enjoy...!
Mac Pro Cube
48 P cores / 6 E cores / 96 GPU cores - CPU & GPU Chiplets on interposer / System in Package (SiP) design
HBMnext Unified Memory Architecture - 64GB / 128GB / 256GB / 512GB
NVMe SSDs (dual NAND blades) 2TB / 4TB / 8TB / 16TB
Eight USB4 / TB4 ports (plenty of expansion ports for external drives, displays, a/v interfaces, control surfaces, etc.)
Two 10Gb Ethernet ports
One HDMI 2.1 port
Four MPX-C slots (for use with asst. MPX-C expansion modules)
Apple MPX-C Expansion Modules
NVMe RAID Storage Module (Quad NAND blades) 4TB / 8TB / 16TB / 32TB
GPU / GPGPU Module (96 cores)
FPGA Module (audio/video)
Neural Engine Module (AI/ML)
The perfect personal workstation for a wide variety of tasks...!
Imagine the below, big flow thru heat sinks on all the needed bits, massive 200x30mm Noctua fan on the front, chugging air thru the system; just like the three fans on the current Mac Pro do...
A modified pic of the 2019 Mac Pro: