SoC(s) on riser cards would allow SoC & RAM upgrades, replacing the whole riser card with a paid exchange from Apple; like upgrading a Silicon Graphics workstation worked "back in the day"...
Six slots because the "in-between" slots in the "MPX area" are removed, they would be blocked by the 4-slot heat sinks from the two ASi (GP)GPUs anyway...
Probably keep the two MPX bays (but not as MPX). It is definitely slot 8 that is in danger. That's where the Thunderbolt controller was . If the TB controller is now inside the SoC then that SoC needs to be near where the old ports where. Probably better if they just put at least 4 TB ports there and just make the 'top'/'fron't ports USB4. If soak up slot 7 also can also do another couple TB ports , HDMI , and some USB ports to along with those. Slot 8 has special GPIO and USB connector on it also ( that regular x4 cards wouldn't 'see') that whole thing goes away.
So the riser card could fill the holes allocated to both slot 7 and 8. The formerly quite tall horizontal (in tower configuration) heat sink for the very large package is not going to get turn 90 degrees and go vertical. The space where the old CPU package was in 2019 model is going to move 'down' in the tower to give the heat sink more clearance. Going 'down' is likely going to minimally run into slot 8. And if want to give more clearance for a J2i bracket moving down to slot 7 can at least get some 'clean air' on at least part of the J2i bracket's top device (i.e., not pre-heated by SoC package). RAM, SSD module sockets , nominal ports driven by SoC controllers .... all on the riser card.
Nominally the classic place for the Afterburner card was slot 5. If that is in the SoC now also , then what would have been pushed from slot 5 to slot 7 can now just go in slot 5.
[ and if Apple keeping the USB/SATA internal connectors could move those up closer to where the J2i bracket so don't need quite as long cabling to get to the drives. ]
The layout for slots 1-6 can stay the same.
The crazy 400->500->700 W trend on GPU cards means probably do need two 4 double wide slots left after finish here. There is going to be no shorter of cards with monsterous heat sinks in the "Compute accelerater" market going forward. However, decent chance though the MPX connector get removed from slot 1 and 3 though.
Considering I had the base Tower with a M3 Ultra and the base Cube with a M3 Extreme, yes...
The TARDIS aspect of getting that Extreme SoC on a Studio footprint is the bigger problem.
Internals would be different than Mac Studio, just the uniformity of the exterior chassis housing is what I want to see with the Cube; Mac mini, taller Mac Studio, even taller Mac Cube...
Even taller Studio ... with one, only one fan, is highly dubious. The lack of proper ventilation in the nominal mini design constraints is going to wreck this thing. ( The Ive the cooling system has to be magically absent philosophy is goofy. It isn't magical at all. ).
Apple would be way, way , way better served to throw that road-to-nowhere-nostalgia R&D money they might throw at that , over to making one (or two) Mac-on-Card systems for hypermodular folks who are going to pick up and leave when can't run Nvidia GPUs and can install their own DIMMs. They could put a Mac inside a slot of their Boxx/Pudget System/Lenovo/Dell/HP box to run the subset of macOS stuff they want to hold onto long term and the rest Windows/Linux. ( as a PCI-e card don't need a AC/DC power supply (get DC power off board). Two TB4 ports , Ethernet , one USB4 port would simpliy the other discrete components on the limited board space. two SSD modules. And pack a fan cooler on top). Mac users could even throw that card into one of the double-double wide slots if wanted another instance to hand off workload to.
That has a way bigger ,
expansive target market than trying to depress higher end Mac Pro sales. Try to GROW desktop aggregate sales . not balkanize them into as small as possible pools of the same size.
Internally, Cube would have vertical logic board up one side & Mac mini-style (just taller & higher power output) PSU up the other with heat sink in between; bottom intake (Mac Studio "foot") and top exhaust via 180mm high static pressure fan...
Again if twerk the orientation of the large, tall heat sink 90 degrees and don't have room ... going to run into placement problems. The Extreme package is bigger but the heat sink is ALSO going to get bigger. It bloats out in 3D; not just two.
What overlap...? For the non-PCIe slot needing crowd the Mac mini goes up to Mn Pro, Mac Studio up to Mn Ultra, Mac Cube is Mn Extreme only...
Missing the point that in the $8K there is hardly anyone up there. The vast majority of the "no PCIe slot" crowd is down in the $2K-4K price space. Dramatically smaller group in the $4K - $7K space . By time get to $8K there is almost nobody left. Pragmatically need to herd those folks onto something they might not totally prefer but will buy because there is no other option; the Mac Pro.
If the riser card path is around in the Mac Pro it isn't just don't need any PCI-e card... also don't never need any RAM/CPU/GPU upgrades ever either. If bought the extreme because needed more RAM what make you then they won't need more RAM later? If folks are on the lunatic fringe of needing more CPU cores , GPU cores, and/or RAM also diving into the group of folks who won't want those all permanently soldered down.
If one needs PCIe slots, there is the Mn Ultra / Mn Extreme Mac Pro tower...
At some point the target market is so small can't keep chopping it into smaller pieces. There are substantial number of old school hypermodular Mac Pro users who are going to walk away. Pretty likely the eligible pool of users is going to shrink smaller. Taking something that is already small and making it smaller isn't going to help much.
When the Mini got a M1 Pro option that was expansive. The Studio is probably somewhat expansive of the old iMac 27" market. The big CPU/GPU iMac folks herded into Studio. Some of the xMac , "I'll never buy an integrated screen" folks into Studio. And pulls some Mac Pro folks down into Studio. (or opened door back up to them after closing it by increasing the entry Mac Pro price 100%. folks in the 3K-5K zone of old Mac Pro).
The Cube thing isn't going grow nothing. Most of the folks probably want to point to were previously herded into buying a Mac Pro and were happy enough to pay/use it. That will still be true if don't try to balkanize what is left of the Mac Pro user space. [ the very unhappy folks probably left for Windows/Linux. And at $8K price point not going to bring many of those back. If have an insatiable core count or insatiable GPU core count problem ... $8K is going to buy more there if relatively price/cost constrained ]
Finally, folks who really need an Extreme level of processing power probably need more than just one internal disk drive. The notion that most of these folks are not going to put another PCI-e SSD inside their system is likely bankrupt. That much processing power likely leads to high data capacity storage needs . Apple's SSD pricing is quite high. More money for Mac Pro can be offset by far cheaper market rates for your high capacity storage needs.
Folks can leave lots of their data on SAN/NAS networks , but those don't have to be capped at 10GbE either.
The Mn Extreme Mac Cube would very much so be a "LOOK AT ME" halo product, Phil Schiller would return just to drop the "can't innovate my ass" line again; and when paired with a 16" iPad Studio & Mixed Reality Headset, no worries about monitor "shadow" placement issues...?
The Extreme's SoC major problem though is that is far , far , far more needs BUYERS not just lookers. Need to grow the desktop Mac market bigger so that it can get foundation so it can econoically justify an appropriate die design that is effective.
The whole thing of Readily Pro as a primarily replacement for a mainstream , primary display is ergonomically and battery challenged hot mess. Besides, how is it going to work any significantly better with a Cube than it would a Mac Pro? Give up using 27-32" monitor so can use 16" ipad. Yeah OK.
If Apple wants a box to sell to the "I'm an Apple super duper fan", hipster crowd for conspicuous consumption then just come out with a model of the Studio Utlra that is painted Space Grey and has one extra 'exclusive you don't have one' GPU core activated. It would be far less cheaper R&D and less of a distraction.