I don't think Apple Processors and PCI lanes are a very real idea,
The M1 and M2 have four PCI-e v4 lanes. So it is real. Just not in bundles of x16 lanes, but Apple does have a working PCI-e controller(s) on the chips. 10GbE sockets on the Studio only needs x1 PCI-e v4 lane (or two run at v3 speeds).
Apple supports PCI-e cards over Thunderbolt. For example, two recent cards introduced by Sonnet Tech:
View videos and tech specs for McFiver PCIe card, and determine hardware and OS compatibility.
www.sonnettech.com
https://www.sonnettech.com/product/m2-2x4-ssd-low-profile-pcie-card/techspecs.html#techspecs
[ Not listed at the 'top' of the spec sheet in the "Mac" section , but in the Thunderbolt section there is a Mac subsection there. Both cards work with 'M1' Macs. ]
What Apple is showing zero progress on is PCI-e card GPU drivers. PCI-e in general though isn't an issue. There is classes and features in the driverkit framework for modern Mac drivers for a general class PCI-e card. It is all very real because there is real product being sold right now.
Since Apple is doing work for Thunderbolt PCI-e there is little good reason for them to ignore PCI-e for a dual or quad die Apple SoC. Even if they blindly just took the "Max" class die and re-packaged it up as a "Ultra" and "Quad/Extreme" there would be at least one "extra", unsued x4 PCI-e controller just lying around ( and for four dies that could easily bloats up to three (maybe two if still get two 10GbE ports). ). One or two Plex switches from x4 PCI-e v4 to x8 PCI-e v3 and would have a natural fit with those two Sonnet cards.
There is a 'loud' minority who equate "PCI-e" with "discrete GPU cards". Those are suppose to be same thing (without GPU cards , PCI-e wold collapse into a heap of uselessness. ). That isn't really true either.
The Apple SoC is highly likely to start off with an "Utlra" class SoC and pretty likely to include something bigger than "Ultra". Given the current "Ultra" already has under utilized PCI-e controller then it pretty likely that a M2/M3 future version likely would also. So zero slots is pretty unlikely. It is already provisioned, simply need a bigger enclosure and simple switch to widen out the lane bundle. That is no where near a "moon shot" product.
Similarly, if Apple is just lazily duplicating the laptop dies then there also will be a glut of Thunderbolt controllers. If super lazy then just run that to a embedded Intel discrete controller and can pop out a x4 PCI-e v3 slot(s) also. Six TB ports is dubious ( two on front and four on back is more useful than six on the back. Studio Ultra only goes to six (with two unused TB controller.). If crank up to four laptop dies there would be even more idle TB controllers.
A desktop die that traded TB port provisioning for PCI-e lane cluster provisioning could more directly get x8 slot infrastructure without the discrete switches and gyrations. Apple probably wouldn't chase PCI-e v5 in short term (zero good Mac laptop die tie-in there. ). Apple doesn't need to radically change from their laptop die constraints and infrastructure, but could be laid out better to scale to multiple die solutions and still get to more PCI-e lane provisioning without any new basic building blocks.
and AMD cards easily shame any M1 or M2 in real work performance,
The very top end of the scale. But the 6600X , old 580X , W5700 , W6400 . Those are not out of reach for a "Ultra" or "Quad/Extreme" class M2 generation SoC. The Mac Pro isn't going to have a 'plain' M2 GPU. Or likely not a M2 Pro/Max class GPU either.
The "floor" of minimal RAM is likely going to be a bigger problem for many of the very "old school" Mac Pro users than cranking the RAM even higher. Like the entry level price doubling from MP 2013 to MP 2019. There will be folks "left behind".
and component expandability are opposed ideas to the SOC principle.
As pointed out above.... it won't be hard to provision some PCI-e slots on the new Mac Pro system even with Apple's current SoC design parameters. ( new USB ports , SSDs , Audio, Video capture , etc. are all covered by macOS on M-series drivers now. Zero good reason why a new Mac Pro would be excluded. )
So at this point im not really sure of what to expect from a new Mac Pro.
It is not going to be the ultimate commodity component container box. The MP 2019 default SSD is technically modular but it is also not a commodity component either. Probably more for the next iteration. RAM and GPU probably not ( Nvidia and AMD GPUs don't have modular RAM either. ) AMD/Nvidia are heading for the 400-600W high end GPU card range and Apple is not. Every M-series presentation Apple has made so far is not about consuming the highest wattage possible at all.
Apple is also highly unlikely going to abandoned the Audio/Video capture card market for the Mac Pro. Extremely likely going to limit the Mac Pro to one, and only one, internal storage drive.
The rumor of a "half sized Mac Pro" has a decent chance of being true. It won't be a 1400W, maximum USA wall socket outlet system. Probably get something that beats the most popular configuration of a Mac Pro 2019 ( 16 cores and W5700 ) with significantly less power consumed and still can hold some PCI-e cards.
It probably won't be a HP Z8 or Dell 7820 "killer" system though.
