2020 seems prime for redesigned iMac with slimmer bezel, SSD-only, redesigned cooling.
Like 16-inch MacBook Pro before it, I can see iMac switching to different sized screens, perhaps 24- and 32-inch screens with higher resolution.
With new iMac getting improved cooling, perhaps iMac Pro isn't necessary anymore?
iMac Pro has a Xeon processor with higher-end config offering 18 core count and 256 GB maximum RAM. New 10th gen non-Xeon i9 X-series maxes out at 18 cores and 256 GB maximum RAM as well, without Xeon features like ECC RAM and higher maximum # of PCIe lanes.
With Mac Pro filling the Xeon processor gap, I think Apple can eliminate iMac Pro altogether. Offering i9 X-series, 256 GB RAM, 8 GB SSD, pro GPU, and space gray color options should be enough.