My speculation and wish listing:
iMac Pro - I think they might discontinue the 8 core base config, and drop the price on the rest, making the 10 core the starting config. Similar to how they dropped the 4 core trashcan towards the end of its life. Then update it properly when the new GPUs and/or Mini LED is available.
21" iMac - Replace with 23". Maybe keep an entry 21" around?
27" iMac - This is what I am interested in, and am ready to buy if I like what I see. I suspect we won't get a full redesign unfortunately. I'd love to see a bigger screen around 30' in the same size chassis, but I would be ok with the same design, with a 10th gen 6-8 core CPU (with a 10 core BTO option), 16GB base RAM (user upgradable), SSD only, T2, a decent GPU (5700/5700xt) and decent cooling (like the iMac Pro). If Apple comes up with something close to that I will likely pull the trigger.