I suspect (from reading a lot of Mac blogs) that we're looking at something like this - this is the machine I've posted about on MacRumors - others who read the tea leaves differently have posted other possibilities:
CPU: New Cascade Lake Xeon-W, 12 to 28 cores. These $1000-$3000 CPUs are cut-down versions of Intel's Xeon-SP server CPUs with the multiprocessing features removed. The advantage over the smaller LGA-2066 Xeon-W chips is that they can have more cores - Apple will offer options that exceed the 18 core maximum of the iMac Pro (which uses the smaller chips).
RAM: 6 channels, 6 or 12 slots, minimum 48 (6x8)or 96 (6x16) GB, maximum 384 (6x64) or 768 (12x64) GB. 8 GB DIMMs for these CPUs are substantially more expensive per gigabyte than 16 GB DIMMs, so Apple may start with the 96 GB configuration that's only a little more expensive.
Storage: Primary SSD array run from the T2 (or T3?) controller. Either PCIe x4, two channels (fast, 1 TB to 4 TB configurations) or PCIe x8, with four channels (really fast, with 2 TB to 8 TB configurations). The iMac Pro uses PCIe x4 with two SSD channels off the controller, and the Mac Pro will either be the same or a doubled version. T
There may also be a couple of standard (NVMe) or proprietary storage expansion slots, since the SSD run off the T2 is locked to the T2 chip and can't be expanded without reprogramming the security (Apple could give their stores tools to do it, but a user won't be able to). Right now, they won't expand the iMac Pro's T2 locked SSD at all, but they can replace it without replacing the motherboard, which means that the reprogramming tool exists somewhere.
GPU: AMD GPU in a proprietary Apple GPU Slot, probably with a second Apple GPU Slot free. The Apple GPU Slot is probably just a PCIe x16 slot with a small extension to pump the video out over Thunderbolt, but the size and shape of the GPU card will be nonstandard (including no rear panel ports). There are two reasons to make it nonstandard - one is that standard PC GPUs are inefficiently (and loudly) cooled with their big ol' fans. The Apple GPU Slot will use the computer's cooling (possibly liquid cooling) instead. The second is that Apple doesn't want to deal with NVidia GPUs, and an incompatible slot keeps them out.
Depending on the timing of the Mac Pro versus the timing of Navi, the baseline GPU will probably be a Vega 64, with Radeon VII and Instinct models as upgrade options. If Navi's ready, they'll use comparable Navi GPUs A second GPU will be optional, and will probably have to match the first. They will probably commit to offering GPU upgrades (but they said it was possible for the Mac Pro 2013 and the only upgrades that ever shipped were the higher end GPUs offered as BTO at release).
Ports and upgrades:
RAM should be user upgradeable and there will probably be a way of adding internal SSD storage (although upgrading the boot drive will at least require an Apple technician if they allow it at all, due to the T2). The CPU may well be unofficially upgradeable until Intel stops using the LGA-3647 socket (probably using other CPUs that Apple offers, possibly some that they don't). There is a slight chance the CPU is officially upgradeable as well. The GPU will probably be officially upgradeable, although parts availability may be an issue.
There may be a free standard PCIe slot - probably half length and low wattage - primarily for I/O boards of different types.
The primary port will be a bunch of Thunderbolt 3 ports (this is also why AMD CPUs are unlikely - they don't support Thunderbolt easily).
The most likely configuration is 6 ports on 3 buses (like the 2013 Mac Pro, but Thunderbolt 3 instead of 2). Either 4 ports on 2 buses (iMac Pro) or 8 ports on 4 buses would also be reasonable. There may be a way of bridging two Thunderbolt ports on different buses to support high-speed external PCIe boxes.
There will almost certainly be dual Ethernet ports, at least one of them 10 Gb. The most logical configuration is actually one 10Gb port to connect to the fastest things on your LAN (NAS units, servers, iMac Pros, MacBook Pros using TB3 to 10GbE adapters) and one 1 Gb port for Internet connectivity, printers, etc. There's no reason to go 10 Gb on the secondary port (since segmented networks where both segments are 10 Gb are rare - the usual reason to segment is to get the printer, etc. off the 10 Gb segment) , but Apple might, just to simplify things - the secondary port will just auto-negotiate 1 Gb speed, so there's no reason not to.
There should be a few convenience ports - the most obvious is at least a couple of old-style USB-A ports, hopefully up front since the major reason to have them is for thumb drives. They will probably include a headphone jack, also for convenience sake. I personally think card readers are unlikely - SD will probably not be the only standard for much of the life of the machine. From a still photographer's viewpoint, SD plus XQD/CFExpress would be a good combination, but video folks use three or four others as well.
It'll be expensive (think iMac Pro, then a little more), but I don't think it'll be as expensive as some people on the Mac forums are saying (forum opinion ranges from $2999 starting price to $15,000 starting price). Intel did us a big favor with those new Xeon-W chips that are cut down versions of the server chips. I used to think it'll be a $6499 machine because it had to use a server chip - now I'm closer to $5499 to start.
$5499 Mac Pro: 12 core Intel Xeon-W, 1 TB SSD, 48 GB of RAM, Vega 64
or maybe it'll be $5999, but with a higher base configuration? 12 core Xeon-W, 2 TB SSD, 96 GB of RAM, Vega 64.
BTO options:
CPU: 16 core $500, 24 core $1500, 28 core (also higher clocks) $3000
GPU: Vega 64X $200, Radeon VII $500, Radeon Instinct $3000+
Second GPU: Has to match the first, Vega 64 $400,Vega 64X $900, Radeon VII $900, Radeon Instinct $3400+
RAM: 96 GB $300 (if not standard), 192 GB $2000, 384 GB (gulp!) $7000
Storage: 2 TB $400 (if not standard), 4 TB $2000, 8 TB (if offered) $4500
The base model is a desktop workstation that fills some high-end photographers' needs, especially if they also do video. The smaller upgrades are relatively reasonable (a $6700 machine probably won't break the bank if $5500 is OK), while the top upgrades are for Hollywood. It tops out over $25000, but that's not for photographers...