What would you like to see in an iMac Pro?
...an aux DisplayPort input so you could use it as a display for other computers. Otherwise, no sale on an expensive display panel that you can't carry over to your next computer.
When the iMac went 5k in 2014 there was an
excuse for dropping Target Display Mode because there was no current interface standard that could support 5k over a single cable (DP 1.3 was approved the same year, but wasn't in the shops) & the iMac used a custom internal interface. Today, DP 1.4, which goes up to 8K, is current (and implemented in Macs) and by the time a hypothetical iMac Pro comes out, DP 2.0 (8k+ with all the trimmings) should be around, so, no excuse apart from penny pinching & planned obsolescence.
Anyway, I don't think the iMac Pro is going to happen - a Mac Studio Max + Studio Display costs the same as a 2020 i9 iMac with the 32GB upgrade and a 20 core Studio Ultra + display is a
lot cheaper than an 18 core iMac Pro used to be - and a lot of "pro" customers will prefer being able to choose the display independently of the computer (there's already a choice of Studio Display, Pro XDR or a zillion 3rd party possibilities),
The gap in the Mac range is for something to replace the old starting-at-$1800 5k iMac options - which put the notional cost of the 5k display at < $1000. That was always a bargain (
if you
wanted a 27" 5k) and I don't think that is sustainable (at a profit margin sufficient to get Apple out of bed) now that 5k displays are established as a Mac-only niche.
Maybe we'll see a new iMac when Apple have something new to offer display-wise at that size (miniLED etc) - rather than a warmed-over version of 5-year-old tech - but that's probably going to be too pricey for a low-end iMac and I really think that the high-end market, once they stop and think, will prefer the Studio concept. Also, with Apple Silicon, there's now minimal difference in performance between desktops and laptops until you get to Studio Ultra level, so a lot of people who previously had an iMac on the desk and a MBP for the road can now just have a MBP and a large display on the desk.
It makes more sense for Apple to produce products like the Studio Display and Pro XDR which they can sell alongside
any Mac model (from the MBA to the mythical Apple Silicon Mac Pro) than a high-end all-in-one for a particular niche. (Speaking as someone who only bought an iMac in 2017 because there was no viable headless Mac option at the time).