I'm afraid it would be like this.
Imagine single block called "module" which is very easy to install, just click and snap. Each module connected with special connector and contains different type of components. Graphics Module, Memory Module, Processor Module. All should be obtained from Apple.
If those happen, it would comes with Apple usual creative tagline such as "Defining a new way workstation" or such. Hope future doesn't sound pessimistic like me, but I'm doubt Apple want make traditional tower computer again (looking at cheese grater)
I'm just want say thanks to iMac Pro because it bring support for Vega GPUs and newer Intel CPUs. Great machine though, but I'm skip.
Instead, how about separate OSX for those workstation, Apple? Honestly I'm start sick with annual cycle of desktop operating system because sometimes new OS break some software and my workflow. It end up usually with not updating immediately or skip entirely new version one year off, except they bring useful features like support for new hardware.
Separate dedicated super stable workstation version of OSX for those machine (iMac Pro, Mac Pro) which get less update, just security patch. Let generic annual MacOS installed in light machine such as plain Macbook / air