Needs to be shorter than 19 inches.
Single / Dual isn't in the workstation targeted line up anymore for either Intel or AMD.
Intel Xeon W / AMD Threadripper are different set of requirements than Xeon SP and AMD Epyc
More than a dual CPU socket they more fundamentally need to open up to having 8 DIMMs. ( which is dump the CPU card model is probably easier and more affordable to implement. )
The generic, long card GPU video card solutions don't cleanly integrate with Thunderbolt. They don't functionally solve the problem. So it is basically form over function to hammer that round peg into a square hole. The vast bulk of the Mac ecosystem has Thunderbolt. The next next Mac Pro will need to integrated into that ecosystem. Apple could work out a new double wide PCI-e+displayPort format ( perhaps something along supersized MXM ) that would be the long term longitudinal solution for Mac Pro primary display GPU cards. If future cards would fit and work. However, as long as the overall market ignores the problem space there won't be a generic "race to bottom" solution market.
A x16 PCI-e double wide slot with power cable for up to ~300W range card. Yes that would be clear differentiator over the iMac Pro.
Two more x4 slots (like the 2010 models ).... doubtful. The 10GbE only really needs about x2 so could split/share like the 2010 models with that.
Between Thunderbolt and x4 SSD options the other is probably going to be assigned specifically to one of those roles.
Just one Xeon SP doesn't particularly make sense the way Apple has done things. HP/Dell/etc sell empty CPU socket systems, but Apple never has. That isn't likely to change at all and probably really doesn't add much for the market share subset that Apple is in. Apple isn't trying to be every possible system for everybody. That is way off the general strategic line.
Intel Xeon W is a repackaged baseline SP design that is more price optimized for higher base/turbo clock and generally better single user workstation performance.
if the 19in rack and Xeon SP is being looped here to drive after Computational clusters ( e.g., 8-20 Mac Pros racked up in a HPC render farm ) that isn't going to be the primary , or even secondary , targets. Substantively large groups of Mac where there is no direction interaction with the macOS GUI interface. macOS isn't moving to being a "data center" , headless user focused OS. Mac Pros with primary hypervisor (ESXsi , etc.) , Linux or Windows Server installs is a grossly bad fit with direction Apple is going.
Apple themselves buys lots of that stuff from other people and it works fine for them. I doubt they see the point of doing that themselves with a Mac product.
Don't need that if have an open x16 slot. There are already pseudo proprietary expansion box solutions for an empty x16 slot out there. Apple doesn't need to do one themselves.
