A noted leaker has been offering up a wealth of details about a potential 2022 Apple iMac Pro that would be fitted with a choice between the M1 Pro and M1 Max processors. The supposed iMac Pro with Apple Silicon would also sport an HDMI port and possibly be priced at a surprising level. However, it seems only a medium-sized 27-inch option is on the cards right now.
https://www.notebookcheck.net/Alleged-27-inch-Apple-iMac-Pro-specs-M1-SoC-options-release-date-and-likely-price-divulged-in-weighty-leak.576410.0.html
Unlikely for the iMac Pro. The regular iMac, much more likely, will be available with the M1 Pro or M1 Max in the standard configurations, and then another variant for the top-end standard iMac.
An iMac Pro will yet need a chip capable of more RAM and GPU power than the current M1 series delivers.