Let's not forget that the regular iMac is coming up to 1 year old in March. Although the CPUs may not get a refresh if Apple refresh in March (9th generation CPUs aren't that much of a boost over the 8th generation) I think Apple would use AMD 5500/5700 GPUs and given the drop in price of NAND storage they could actually go all SSD and get rid of Fusion drives.
If they delay the iMac refresh until October 2020 then Comet Lake S (another 10Nm iteration) could come into play with Hyperthreading on i5 as well as i7 CPUs. This would be seen as a significant boost in performance but would also bring the iMac benchmarks closer to the iMac Pro.
The iMac Pro CPUs have been superseded as mentioned in this thread already. Apple would need to engineer a new motherboard too and we'd obviously get the powerful AMD GPUs and maybe more storage for the money as standard seeing as Apple seem to have got good deals on NAND if you look at the MacBook Pro 16" specs (double the storage for no additional cost).
For me, I'd like to see Apple introduce a lower entry iMac Pro to replace the top SKU 27" iMac - it would help them justify the cost of re-engineering an iMac Pro refresh.
The forthcoming Comet Lake S CPUs
allegedly start with 6 cores/12 threads as i5, through 8 cores/16 threads as i7, and then 10 cores/20 threads on i9. This could, in theory, see an appropriately specified iMac able to compete with the 8 and 10 core versions of the iMac Pro.
If they go all SSD on the 2020 27" iMac they could just as easily kill off the iMac Pro and use the same engineering solutions to go with Comet Lake-S (up to 10 cores/20 threads) with the i9 and AMD 5300/5500/5700. Obviously there would be an issue with locking away the RAM, but it's a good time for Apple to consider making 16Gb of RAM standard on the 27" if they have to lock it away.