i would like to see Apple go 24” and 30” by returning to the 16:10 aspect ratio, becoming pixel doubled equivalents of the old Apple Cinema Displays of those sizes so that they are retina. The 24” would also be a retina equivalent of the old 24” iMac, not a coincidence as iMac sizes have often followed previous Apple monitor sizes, hence why I have some hope they do again.
24” 3840x2400
30” 5120x3200
Apple have gone with the commodity prices in this case. They can afford to order 16:10 screens for laptops but the panels available for iMac largely no longer exist or would significantly drive the price up - something that Apple seem to be sensitive about for their desktop line.
Apple could consolidate the regular iMac line within one panel but that would have to be something like a 24-25" screen which leaves the iMac Pro users rocking the 27" 5k screen in my opinion.
Such a 24-25" 4k panel would inevitably have commodity compromises for cost - just look at the 23.7"
LG UltraFine that Apple are stocking now. The only exception I could see Apple making is to request a 25" variant of this at the same dot pitch so they could offer a true 4096x2304 display.
They'd probably draw the line at 60Hz rather than try and source a version that could do 'Pro-Motion' 120Hz or otherwise call it a range topping 'Pro' variant.
I was actually referring to using a 32" 8K panel (like the one in the Dell) for the iMac even though Apple offers a 32" 6K panel in the Pro XDR. So I was saying the 8K panel might be higher resolution, but the 6K panel is better in every other category so Apple could, in theory, use an 8K panel in the iMac line and still offer the 6K Pro XDR monitor for the Mac Pro because the XDR monitor is better in every other way. However, I believe that the 8K panel is too expensive for the iMac and I also believe Apple might feel 32" is too large a screen size for the iMac.
Like you say, we're getting into the realms of technical difficulty expecting a 'prosumer' iMac to be able to drive an 6k or 8k display. The Dell might be more mainstream but who would buy such a unit in an AIO rather than as an external monitor? The main problem, rather than size, is the cost which would be prohibitive.
Personally, I do not believe any 4K or 5K iMac should ship with only a spinner. So I would like to see Apple offer the base 27" 5K iMac at $1799 with a 256GB SSD or a 2TB Fusion Drive and the base 21.5" iMac 4K at $1299 with the 1TB Fusion Drive. That way every 4K and 5K iMac has an SSD (32GB/128GB/256GB) as part of it's storage configuration.
I'd keep the 1920x1080 with a 1TB HDD around (and drop the price to $999), but not as something the general public can buy. Make it Educational and/or Business only where the people buying it understand the limitations it has in that configuration (and who can upgrade to a Fusion Drive or small SSD if they feel it is prudent).
If Apple want to introduce the T2 then spinning hard drives will be history, a base configuration of 256Gb SSD isn't that much more expensive than the 1Tb Fusion drive.
As you say, Apple may wish to leave some variant of the 2019 21.5" as the entry level machine - inevitably with a spinning hard drive - they already do so with the non-retina base 21.5" model.
But there can't be any doubt that the following things will have factored into an iMac refresh:
1. Display Panel choice could be dictated by marketing and panel pricing both now and for the next 5 years.
2. Introduction of T2 means the end of HD and Fusion Drive - thinner case design possible
3. Professionals wanting VESA mount option - less weight desirable?
4. Is removable RAM back on the menu for a redesign that could cover a very expensive top SKU price?