A friend has an iMac Pro, and it is a screamer on his 500,000 image Lightroom library. The new Mac Pro is likely to be even faster, but also even more expensive.
From reading the Mac forums, here's my guess as to the rough specs of the new machine (nothing more than the musings of a person who's been using the Mac for 35 years - this is NOT based on inside info).
It's going to be expensive - my best estimate is $5999 or $6499 for a (very nice) base configuration. Apple has always preferred to sell all-in-one computers (iMacs), and they protect the iMac market by pricing the Mac Pro above any usual iMac configuration. They will probably only take the base price of the iMac Pro into account - I can't imagine them starting the Mac Pro above the top CTO prices of the iMac Pro!
It's not going to be as expandable as some people wish for - specifically, it probably won't be compatible with standard PC graphics cards (Apple hates NVidia), nor accept internal spinning hard drives. It probably will have replaceable graphics, and there may be single and dual graphics options. There will certainly be multiple AMD GPU choices available, and there may be the option of installing two - but they won't be commodity cards from Newegg.
I strongly suspect it'll use the "big-socket" Xeon processors - the recently released 28-core Xeon W-3175X ($3000 for CPU alone) or a close relative will be the top processor option. They'll probably use something around a 12-core in the 3.2-3.5 gHz range with a turbo near 4 gHz as the base option, then offer three upgrades that increase core count at similar per-core speeds. The other possibility I wouldn't rule out is AMD Threadripper and EPYC (they won't offer both Intel and AMD options, but either one could be their choice). They won't be standard Threadrippers, because Threadripper uses non-ECC RAM and tops out at 128 GB - EPYC will go (much) higher, and so could a slightly modified Threadripper Apple has the clout to get. IF they use AMD, the CPU upgrades will probably be a little cheaper.
12 cores - base
16 cores - +$700
22 cores - +$1500
28 cores - +$3000 (typical Apple tax - they'll sell a $3000 processor for $3000, but not give any credit for the $800 12-core they take out).
They may offer some dual CPU 22-core and 28-core per CPU options at the very high end. Dual processors with 16 cores or less make no sense, because a single higher core count processor would outperform them. A dual 28-core would be at least an $8000 upgrade option from the base model.
The big-socket Xeons have 6-channel memory, so the memory options will look odd to people used to 4-channel configurations. The motherboards have either 6 or 12 RAM slots, and configurations that fill 6 or all 12 are much faster than configurations that leave empty channels. Apple very rarely sells configurations with empty channels, so 48 GB is the minimum practical configuration. RAM is almost certainly expandable after purchase, probably by the user, so Apple-taxed RAM may not be the only way to go. If they're using AMD, it'll be 4 and 8 channel RAM, but the costs per GB should be similar.
48 GB - base
96 GB - +$1200
192 GB - +$3600
384 GB - +$7000 (highly uncertain)
These RAM prices are based on the iMac Pro. Since RAM is user-expandable, Apple may go a little easier on users?
They will have multiple AMD graphics options, with a base that is either the Vega 64 from the iMac Pro or a successor model. They'll almost certainly have a GPU upgrade in the $500-$1000 range, and probably at least one more, which could be two of the upgraded GPUs for $1500 or so, or it could be one of AMDs Instinct cards. They may very well have multiple upgrade options, and I wouldn't be surprised to see a $5000+ upgrade available.
I suspect they're going to start high, and offer options running VERY high, on SSDs. The boot SSD will almost certainly be run through the T2 (or possibly T3) chip, although they may offer some M.2 slots that accept standard PCIe SSDs for storage (no boot unless you disable some security).
My suspicion is that they'll start at 2 TB, and offer 4 TB, 8 TB and maybe even 16 TB options. They might start at 1 TB in a $5999 base configuration ($750 upgrade to 2 TB, add $750 to other options, there's probably no 16 TB option)
2 TB - base
4 TB - +$1500
8 TB - +$4000
16 TB - +$9000
Most Mac Pros sold will probably be in the $7000-$10,000 range, although there will be combinations of options that push it over $20,000 - possibly way over $20,000 if dual-CPU and dual-GPU options are offered. The highest-end models will sell in very small numbers, probably mostly to Hollywood.