This is a M1 Max SoC on the 16" 2021 MacBook Pro motherboard.
So looking at this, we can see what might be needed, size-wise, for Mac mini / iMac (Pro) / Mac Pro Cube / Mac Pro units?
Motherboards for the various chassis options will be different?
I kinda feel the rumored 27" iMac Pro will only be a "Pro" model because it will have a choice of the M1 Pro or M1 Max SoC; and named such ("Pro") to match up with the new 14" & 16" MacBook Pro laptops. The same with the M1 Pro / M1 Max powered Mac mini, here forth know as the Mac mini Pro! ;^p
But the Jade 2C / Jade 4C units, those will be the Mac Pro Cube & the Mac Pro. Low volume (sales-wise) SKUs for the low-volume dual & quad SoC SiPs.
Looking at the mobo from the 16" MacBook Pro above, I would say the most a Mac Pro Cube could hold would be two M1 Max SoCs, here forth know as the M1 Max Duo! See post #131 above for more on a proposed Mac Pro Cube chassis & cooling!
As for the larger, towering (see what I did there?) Mac Pro, an easy fit for a M1 Max Quadro loadout!
This leaves the mobo for the Mac mini, looks like a single M1 Max SoC mobo should fit just fine; just hope Apple doesn't make the Mac mini thinner "just because". FYI - I believe the Mac mini sticks with the 7.7" x 7.7" footprint because it is a tribute to the G4 Cube (7.7" x 7.7" x 9.8") and therefor an ongoing tribute to Steve Jobs! ;^p
If anything, Apple can make a M2-powered Mac mini the new thinner (shorter?) rumored model, but make the Mac mini Pro a bit taller (this might be the 3x taller rumor?) to allow for more cooling capacity, with 2019 Mac Pro vent holes integrated somewhere (?), available in Space Black!
TL;DR:
Mac mini Pro - M1 Max SoC - new taller chassis / available in Space Gray
iMac Pro - M1 Max SoC - new chassis like the 24" iMac / available in Space Gray
Mac Pro Cube - M1 Max Duo SiP - new "retro" Cube chassis / available in Space Gray
Mac Pro (tower) - M1 Max Quadro SiP - available in Space Gray
And a return of Space Gray keyboard, mouse, & trackpad options.
EDIT TO ADD:
If Apple were to offer a larger screen iMac (30" or 32"), this could be the real iMac Pro, with the 27" model being just a 27" iMac (assorted colors like the 24" iMac) and the 30"+ iMac Pro (Space Gray) could have the M1 Max Duo SiP?