The iMac and iMac Pro both need an update, with the 16" MBP uncomfortably close in performance (not to the many-core versions of the iMac Pro).
Right now, the MBP is significantly ahead of all iMacs except the 9900K and the Pro in any benchmark you can name. It's very, very close to the 9900K and the base Pro (within 10%), and it is the cheapest of the three when you equalize configurations. The 9900K looks cheaper than the MBP, but once you get a comparable-size SSD in it and replace the ancient Polaris GPU, it's not.
Apple could either do a minor redesign or a major one... If relatively minor:
The 27" will stay similar in appearance and lose the spinning rust - SSD only and probably the super fast T2 driven variety. 512 GB standard, 1 TB in upgraded model, 4 and maybe 8 TB CTO.
The space saved will go into some version of the iMac Pro cooling system
New Comet Lake CPUs, topping out at a 10-core.
Hopefully 2 TB3 buses (4 ports)
10 GB Ethernet option (or standard)
Navi GPUs (fairly small Navi standard, some version of a 5700 available as an upgrade option).
The iMac Pro will get a CPU bump to the new Cascade Lake Xeon W series and a Navi GPU bump instead of Vega (same design). To differentiate it from the "standard" iMac, only 2 CPU options - either 12 or 14 core (probably not both) and 18 core. Priced about like the 8 core iMac Pro for the 12 or 14 core model, like the 10 core for 18 cores. Standard SSD 2 TB, Navi GPUs with the "little" option just a bit bigger than the upgraded GPU in the standard iMac, "big" option whatever can fit in the cooling envelope.
If they go big there are two options. One is a redesign while staying Intel, the other goes to AMD CPUs.
If they stay Intel, it'll be the same Comet Lake CPUs above in the 27", because they're the only choices.
The only real difference in specs from the above is that it'll be a new chassis, lose the bezels and at least part of the chin. Styled like the XDR display.
The iMac Pro goes to 32", 6K (detuned version of the XDR display).Styled like the XDR display. Same specs as above (12 or 14 and 18 core Cascade Lake Xeon W), except that the GPUs are bigger because the 6K display needs it and the extra room allows for plenty of cooling. Gains 8 RAM slots that the processor already supports, allowing for a reasonably economical 256 GB RAM configuration and a very expensive 512 GB configuration. Standard SSD goes to 2 TB, gains 8 TB and maybe even (very expensive) 16 TB options.
The really new possibility is AMD. This requires AMD-compatible Thunderbolt 3 (which exists, but is very new, with kinks still being worked out). If they go to AMD, they probably redesign everything, likely inspired by the XDR display. Any redesign gets rid of the spinning rust.
21.5" gets relatively low-power Ryzens up to 8 cores, with smallish (possibly integrated) Navi. SSDs start at 256 GB (with the higher CPU option starting at 512 GB of SSD). Options to 2 TB, RAM starts at 8 GB, options to 32 GB.
27" gets big Ryzens up to 16 core 3950X (probably something like 3700X, 3900X, 3950X), with medium Navi GPUs. RAM starts at 16 GB, options to 128 GB. SSD starts at 512 GB (upgraded model starts at 1 TB), with options to 4 or possibly 8 TB, (8 TB option is frightfully expensive, but 4 TB is somewhat reasonable).
iMac Pro becomes a 32" 6K machine, gets Threadrippers (24 and 32 core options - probably not 64 core - both because of the price and because it would be faster than any Mac Pro). This is a significantly thicker machine, because it's cooling a 280W Threadripper and a big GPU. RAM 32-256 MB (Threadripper doesn't support 512), SSDs 2-8 TB (maybe frightfully expensive 16 TB).