What Apple should do and what Apple will do are two different questions. I think most of us agree that Apple should provide more expandability than they're actually likely to...
What I think Apple should do:
Processors: Variant on just-released Xeon-SP processors that eliminates the very expensive multiprocessing hardware (or Xeon-W chips up to 28 cores) - that's really the same thing - the goal is chips with options up to well over 20 cores, while not paying $10,000 for the CPU. Intel has released several "SP" big-socket processors without multiprocessing, including a ~$3000 28-core, and they're reasonably priced for what they are. Threadripper (or Epyc) is another very good alternative.
RAM: 8-12 user accessible ECC RAM slots. Xeon-W or Threadripper would get 8 slots (4 channels, 2 slots per channel), Xeon-SP would hopefully get 12 (6 channels, 2 slots). Epyc would get 8 slots, but it's 8 channels - all the RAM would have to be replaced at once. A 4 or 6 slot configuration isn't necessarily enough, and 16 (Epyc at 2 slots/channel) is huge and probably unnecessary (although valuable to allow the user to upgrade RAM without removing all existing RAM).
Drives: Boot drive controlled by T2 plus 2-4 NVMe SSD slots.They aren't going to give up on the T2 and encrypted startup disks, but I'd love to see some internal SSD expansion in addition.
The proper place for spinning rust is outside the case. If you need one or two drives, the price differential between internal drives and external housings featuring the same drive is trivial (under $50, probably less than Apple would charge to make the Mac Pro case bigger to accommodate the drives). If you need multiple drives, there are so many choices ((4, 5, 6, 8,12 drives), (RAID 0,1,5,6,50,60), (inexpensive USB DAS, Thunderbolt DAS, NAS (10 GB or not))) that Apple could never design a solution that satisfies everyone. Better to provide all the requisite ports and let the rust spin externally - it can also go in a closet to reduce the noise.
Ports: 6 TB3/USB-C (3 TB3 buses), 4 USB-A (or even just 2 - it's mainly for flash drives and other "quick plug" items - put it in front, possibly with ), dual 10GB Ethernet (or 10GB plus 1GB), headphone jack... I left off a card reader, because there are so many useful types. SD would be cheap to include (the expense would probably literally be the hole in the case more than the circuit), but it's going to be replaced soon, especially in the video world. For photography, the standards are SD and XQD/CFExpress - video uses those plus CFast, SxS and a couple of different ways of mounting 1.8" and 2.5" SSDs. The really elegant solution would be a front bay with options ranging from a blank (or SD-only) module to REDMag, plus several audio-relevant options for people who may not have a camera, but have a room full of keyboards. The catch is whether they'll sell enough to make the modules viable (and at what price).
Graphics (this is going to be controversial): I don't think Apple should use standard PCI-e graphics. PC gaming cards run too close to the edge to be really stable, they're an old legacy form factor that could easily be replaced by something with better cooling (liquid cooling integrated with the rest of the system?) and NVidia can't or won't produce a stable Mac driver (Apple can't, won't or doesn't want to). They'll end up with a zillion GeForces of every shape and size (which they don't want to support) if people can shoehorn them in there. Either they should use a slightly modified PCIe (maybe as little as blank backplanes with an internal cable to redirect the signal out the TB3 ports) that AMD commits to supporting, or they should use a form factor that permits them to integrate the cooling with the rest of the system, but also permits graphics upgrades. One or two GPU (Vega/Navi 7nm) capability.
Slots: 1 PCIe x16 with 75 watts of power - for audio and video interfaces, not back door GeForces. It could also provide an interface for powerful or multiple eGPUs/machine learning coprocessors/etc.
And I think they should license the MacOS in a limited way: This Mac Pro will serve many needs, but some people will want dual processors, or four GPUs, or insist on 3.5" internal bays. Apple doesn't want to do that - fine... License the MacOS to HP, Puget Systems and one or two others. Make it $300/machine (like Windows for Workstations), so it's not useful for cheap commodity desktops Apple doesn't want out there (who cares about a $300 OS on a $25,000 Z8). Restrict the Hardware Compatibility List to keep stability high. I'd like to see license-built laptops, too - more powerful than the MBP 15" (if heavier).
My best guess as to what Apple will do:
Processors: I think they'll get this one "right" (similar to what I'd like to see) - I don't think they'll give AMD as long a look as they deserve, and we'll see some sort of single Intel Xeon with a range of core counts, topping out well over 20 cores. Their processor purchases are defined by laptops, and AMD doesn't offer either the ultra-low power chips Apple needs for the MacBook or the powerful 45-watt >4 core chips the 15" MBP takes.
RAM: I think they'll get this right, too - although I wonder about either gratuitous barriers to entry (RAM on the back of the motherboard or the like) OR only 6 RAM slots (I don't think they're stupid enough to give us only 4, and I hope not).
Drives: I'm almost sure I'm right on the boot drive - very fast, T2 (T3?) controlled. I hope they give us standard NVMe expansion, but I am concerned we might see some sort of Apple SSD Module expansion instead...
Ports: The modular bay is relatively unlikely - they'll probably go with external card readers only or a single SD-only slot. My only other worry about ports is whether they "forget" the standard USB-A up front.
Graphics: Even though I'm happy to see more restricted graphics than many others, I'm still concerned that Apple might go even more restricted (soldered GPU, or some form factor so proprietary that upgrades are unlikely).
Slots: I think they'll go external box only - at best, they'll have extra TB3 buses (4?, even 6?)and a feature that allows joining two TB3 buses together. Maybe they'll have a proprietary port that feeds PCIe x16 to an Apple (or close partner - Blackmagic?) external box.
Licensing: Unfortunately, I don't think they will.
[doublepost=1557273189][/doublepost]A lot of people don't need NVidia (most photographers, video editors who use software that prefers AMD, audio folks). Those of us who don't need it might not want it - GPU drivers are tricky to write and can destabilize systems. NVidia doesn't exactly have a track record of solid Mac drivers...
What I remember back to is Windows 2000 and XP. Windows 2000 was the most stable Windows (as stable as Windows gets) for a very long time. XP came out a year and a few months later, and was a lot less stable until the late service packs. What had changed was game support. Windows 2000 made no concessions to games. Games only ran if they were well-behaved Windows applications (and sometimes not even then), and Microsoft warned customers about that. XP was prettier, and it added support for games that did various odd things to the system (which had worked in DOS-based Windows 98, but in no NT-based Windows prior to XP) - but it was much less stable, even if you weren't playing games. It took at least Service Pack 2 and possibly Service Pack 3 before it was anything like as stable as 2000.
I'm concerned that adding NVidia support (especially for only one limited-market system) will be like Windows XP's game support - it might destabilize things for folks who are perfectly happy with AMD.
I wish Microsoft had continued to offer an "our most stable Windows, but games won't run" option...
