As I understand it, the M1 SoC has the exact same "Firestorm" performance cores and "Icestorm" efficiency cores as the A14 so I don't know what they could do to differentiate them other than add more "Firestorm" cores since an iMac does not need to worry about battery life.
I cannot see them developing a custom SoC architecture specifically for the iMac / Mac Pro. The advantage of leveraging the A-Series architecture for the M-Series is that we will get annual updates - not just to the CPU cores, but also to the GPU cores, Neural Engine cores and Image Processing cores (even if that just means a better WebCam).
Going custom would put us where we were with the iMac Pro's W-Series Xeons. Low volume sales discouraged Intel from updating the W-2100 and when we finally did get the W-2200 series two years later, it was not much better (just significantly cheaper).
I cannot see them developing a custom SoC architecture specifically for the iMac / Mac Pro. The advantage of leveraging the A-Series architecture for the M-Series is that we will get annual updates - not just to the CPU cores, but also to the GPU cores, Neural Engine cores and Image Processing cores (even if that just means a better WebCam).
Going custom would put us where we were with the iMac Pro's W-Series Xeons. Low volume sales discouraged Intel from updating the W-2100 and when we finally did get the W-2200 series two years later, it was not much better (just significantly cheaper).