As a previous (wise) poster said - this is Apple, so it's all just guesses. Here are mine:
Mac Pro is very expensive (~$6499 starting price, certain (rare) options can push it well over $20,000). Of course, you get a lot for that money , but Apple wants a modular Mac to be expensive enough that people don't buy them to avoid the screen in an iMac - they buy them because they need power that no iMac has.
Something like:
Xeon SP - 12 to 28 cores (may have a dual processor option)
48 or 96 GB minimum RAM, 384 GB capacity (perhaps more, especially in dual processor models)
2,4,8 TB SSD options plus a couple of M.2 PCIe slots
High-end AMD Navi graphics (replaceable, but not with a standard PC graphics card - sorry, no NVidia). Dual graphics option possible
iMac Pro sticks around, loses 8 core option (and maybe 32 GB RAM option), adds 22 core option, 256 GB RAM option at top end. May be physically identical, but could become a 32" 8K machine (or something like that). If physically identical, price doesn't change much, but if the screen grows, the price probably will as well.
Something like:
Xeon W - 10 to 22 cores
32 or 64 GB minimum RAM, 256 GB capacity
1,2,4, 8? TB SSD options - non replaceable
AMD Navi replacements for current Vega options
Here's the question-
Does it feature essentially the current screen with HDR and perhaps Adobe RGB (which would be the 2019 iMac screen or a version of the 2019 iMac screen that supports Adobe RGB)
Or is it 8K (or something intermediate like 6.5K) on a 32" screen???
iMac (standard) keeps its two screen sizes, gets a mild redesign. Forum-goers aren't going to like this, but the realities of cooling, and Apple's business decisions to both keep it thin and charge the Apple Tax on as much as possible are that two unpopular features will stick around, while at least one and perhaps two popular ones go away. The bezels and the chin are almost certainly coming back for another round because they add case volume for cooling (if Apple releases an iMac in 2021-2023 with an A-series processor instead of an Intel chip, the bezels and chin might go away then). Taking away the space for the spinning disk and possibly the RAM door leaves room for an iMac Pro-style cooling system.
Something like:
Core i3 (21.5" only) to Core i9 (27" only), 4 to 8 cores (maybe with a dual-core 21.5" at the bottom end) 21.5" will have 4-core and perhaps 6-core options, while 27" will have 6 and 8 core options.
8 GB minimum (21.5"), 16 GB minimum in 27" models. Soldered RAM up to 32 GB in 21.5", while 27" gets slots (maybe not easily accessible) and a 64 GB maximum capacity, with an off chance of a 128 GB option.
256 GB, 512 GB and 1 TB SSD (21.5") 512 GB to (very expensive) 4 TB SSD options for 27". SSD Only on both models. SSD options may be affected by processor choice (cheaper models may not have access to larger sizes, and higher-end processors may come only with larger SSDs).
21.5" keeps Intel graphics, or may get the unusual Intel chips that contain a small Vega on the CPU package. 27" gets Vega or Navi graphics choices that steer clear of the iMac Pro choices (may use on-package Vega graphics at the bottom end).
Screens essentially unchanged, although HDR is a serious possibility.
21.5" prices largely unaffected (256 GB SSD replaces Fusion Drives), 27" gets somewhat more expensive because the Fusion Drives go away (a fast 512 GB SSD is more expensive than a 1 TB HDD). Like-for-like 27" prices stay similar, but the least expensive way to get the Core i9 9900K may well come with a 1 TB SSD and/or 32 GB RAM.
