I honestly don't it'll come to Mac Pro - I see more sense in them using AMD Threadripper for top end machines and ARM for portables that require less processing.
That would mean they'll have to maintain two separate architectures on Macs in the market at the same time which to me just seems extremely unlikely.
Not only would it increase development effort for both Apple and third party developers but it would also create inconsistencies between platforms. Already today there is a wide spectrum of x86 based Intel processors being used in Macs with features some processors have and some do not.
For example some processors have integrated graphics and offer Intel QuickSync which is a video encoder/decoder that is highly efficient and desirable. It can accelerate H.264 and H.265 encoding and decoding while using very little of the normal cores present in the Intel CPU's which saves power and reduces heat while being much faster.
This feature is not present on the iMac Pro or the Mac Pro because these processors do not have integrated graphics in the CPU package and thus do not contain Intel QuickSync. Contrastingly these Macs contain processors which feature the AVX-512 instruction and that is not present on the normal iMac, MacBook or the MacBook Pro's.
This instruction extension can accelerate performance for scientific simulations, financial analytics, artificial intelligence (AI)/deep learning, 3D modeling and analysis, image and audio/video processing, cryptography, and data compression.
These are just a couple of differences I chose because they're easy to convey but if you look at the iPhone for instance it has a dedicated area of transistors for machine learning. This area of compute is growing rapidly as it becomes used for more and more things, it's not just important for facial recognition and self-driving cars.
Eventually your computer could leverage this for better compression of files, better blocking of ads on the web, better searching of files on your computer, better noise suppression on your microphone etc
Since Apple is already pressing forward with this on their A series chips it stands to reason they would want to bring the same innovations to the Mac and that would mean a full top to bottom processor rollout to keep hardware features consistent in my opinion.
This inconsistency on the Intel processors has caused Apple consternation on multiple occasions for instance when they first released Sidecar for the iPad and some Macs didn't support it due to the need for efficient H.265 video encoding. Apple eventually included such an encoder in the T2 security chip as a way to make all Macs (iMac Pro, Mac Pro etc) consistent in this feature availability, something they wouldn't need to do if they made the entire SoC themselves.