So, I have freelanced in about 50 studios since 2010. This is what happened.
Mac Pro 2008-2012
Small Studios would Get one and upgrade it with various bits RAM/Raid - and Upgrade the GPUs every few years.
Large Studios would but the top of the Range - even the RAM but never actually change anything. They might put in another HD if requested.
MP 2013
Studios bought the Mid / High end Dual GPU and mid CPU, They had decent Network shared storage or Thunderbolt.
Ultimately they were very annoyed that the GPU was NOT upgradable. If Apple had upgraded GPU alone, that would have been enough for most people.
MP 2019
Studios ARE upgrading these with NVME raid cards and Upgrading the GPUs. A few have pro audio PCIE cards. And of course the RAM - I have seen a steady upgrade over time - There are so many slots.
Pretty good chance Apple goes back to that 2008-2012 group and
a. tosses the small budget limited studios out of the mix.
b. raises the floor of the RAM and GPU options high enough so mainly have larger up front budget folks with reduced needs for upgrades. Storage and I/O is enabled ( but display GPUs are fixed ).
Where the MP 2013 has more problems was in internal storage than GPU. GPUs get complained about more , but . The GPUs not getting updated in the MP 2013 was partially because there was pragmatically nothing to upgrade to. Nvidia had put themselves on the "don't use list" ( various things that got them on the "really bad partner" list with Apple) and AMD was stuck and trying not to go bankrupt. AMD mainly concentrated on mid-range (Polaris). That would have gotten incrementally better thermals (which Apple's design had self inflicted problems that amplfied that limitation. But AMD "Fury" solution didn't work all that well: relatively trailing edge fab implementation, more expensive, early generational HBM , etc. )
By the time AMD was back to making mid-upper range GPU progress, the iMac Pro was in development. Apple prioritized that first over any MP adjustment.
My own 2019 is now has 16tb NVME - W6800X Duo ( up from the Vega Pro duo ) and 192 GB RAM From the stock 32gb and the Afterburner card. I even got the Rack for normal SSDs!
Apple is supplying "Afterburner" for no extra charge across the whole line up in M2 generation. Not going to be a "Mac Pro" feature going forward. With the SoCs Apple has put a 'floor' on just how low the memory can be:
Mn Max : ≥ 32GB ( probably won't be offered in a Mac Pro)
Mn Ultra : ≥ 64GB ( probably minimal Mac Pro )
Mn 'Extreme/Doubled again' : ≥ 128GB ( upper end Mac Pro)
So if pragmatically make folks go straight to 192GB in the first place then the actual hard requirements to upgrade are actually low ( e.g., the 2008-2012 case where not much movement there for folks who paid up front).
From the 2008-2012 era Apple has more than doubled the Mac Pro entry price. The MP 2019 left a substantial number of those 2008-2009 very limited budget players off on the sidelines. Apple is already charging approximately $2K for an M1 Ultra in a Studio. It is
not going to get any cheaper when move to M2 (and up) and placed in a Mac Pro. Apple has already set the stage for this mulitple die package solutions as being substantially expensive. They are not going to be chasing low budget buyers with these.
The MP 2019 filtered off folks at the old user base bottom end ( entry from $2,300-3,000 --> 6,000 ). Pretty good chance this M-series Mac Pro does some pruning on the other end. ( that Apple walks back from the 53,000 end. Peeling off $10K or maybe even $25K. ). The 1.5GB RAM upgrade drove $25K of that ~53K max BTO configuration. Cap out the memory at 384GB or so and the max price won't be as high. The 24-28 core Intel Xeon options had a a thousand dollar ">1TB " taxes slapped on them by Intel and Apple also. Remove that tax ( which competition with AMD now successful Epyc has done. ) and backsliding on max BTO CPU package price also.
Pretty good chance Apple is going to try to push more 'value' back into the $6-11K price points for the next Mac Pro. Pretty much going to have to because there is much higher competition between Intel and AMD in this space now than there was 10 years ago.
Whatever they do next - it better be upgradable and have similar RAM/storage/PCIE capability.
PCIe/Storage. Probably. there was a rumor of a "one slot wonder" that got replaced with a 6 slot wonder.
Not just storage, they'll be a need for I/O cards for audio/video ingest/output , networking (> 10GbE ) , etc.
All Apple would 'need' is a modified Ultra/Extreme that could provision two x16 PCi-e v4 complexes to feed into a simiplar PLEX switch in the MP 2019 that feeds 6 of the 8 slots.
There are currently already over 50 PCI-e cards that work with macOS on M-series (via Thunderbolt expansion boxes). Not to provision something that is a working ecosystem would be more than kind of crazy. Even if had to resort to hackery of taking "extra" Thunderbolt controller internally to discrete TB peripheral controller and provisioning PCI-e v3 slots out of that. Apple has already done the work here on the software side to enable many PCI-e cards. No rational reason what so ever for the Mac Pro to then punt on taking advantage of that with zero internal PCI-e slot.
If they max out at 328GB then they'll loose some users, but probably not the majority.
They should add back GPGPU compute to minimize some of the losses there. Effectively 2.5 years and no 3rd party display GPU drivers is likely deliberate. Native iPhone apps and a desire for app developments to spend vast majority of GPU optimization budgets on Apple GPUs likely means 3rd party display GPUs probably aren't going to get supported. Nor is a discrete Apple GPU likely because most of those optimization they want folks to closer cater too are precisely to leverage the unique abilities of the iGPU Apple GPUs.
Mac don't boot via standard UEFI. Off the shelf PC market GPU cards a basically a dead end for GPUs that are suppose to display an interface at boot.
The next Mac Pro will probably have a non optional Apple GPU build in. The question really is what is the second GPU for. I think the MP 2013 already illustrated what Apple is likely going to primarily task it with; compute. If there was an discrete , add-in-card 'compute' card that could just be a Mac (or vastly slimmed down macOS) on a standard Mac SoC. (e.g., a Max , Ultra on a card). And the remote compute basically farmed out like it was a networked Mac on a super fast network connection ( e.g. virtual network over x16 PCI-e v4 would be quite fast).
But if Apple incrementally extended the compute side of Metal ( or reversed the deprecate on OpenCL ... or picked up some other open compute API) that could open door for 3rd party GPGPU.
An optional GPGPU card would add more RAM working space. So can be taking pressure off the internal RAM in addition to adding more computing cores.
