Apple will certainly offer an ARM-based MacPro, but I'm assuming it'll be a very different beast - current one maxes out at 1.5TB of RAM and it doesn't seem likely anyone will integrate that much memory on a chip anytime soon ;-)
Memory bandwidth is one key feature impacting M1's performance. When Apple builds an ARM-based MacPro, we can expect something with at the very least 5 DDR5 channels per socket. It's clear, from this, the M1 is a laptop/AIO/compact-desktop chip.
I would expect more, so that cores don't get memory starved. The M1 has 4 fast cores and 4 slow ones. If we imagine an M2 with 8 fast cores, I would expect it to need 16 channels to have the same performance. That's a lot.
Dunno, the M1 CPU package is tiny, thin, power efficient, etc. It's got 4 memory chips inside the package. I don't see any particular reason why a slightly larger package could have 4 memory chips on one side, and 4 chips on the other to double the memory bandwidth and memory size.
However the M1 is already pretty large (16B transistors), upgrading to 8 fast cores is going to significantly increase that. Maybe they will just go to a dual CPU configuration which would double the cores, memory bandwidth, and total ram.
Memory bandwidth is one key feature impacting M1's performance. When Apple builds an ARM-based MacPro, we can expect something with at the very least 5 DDR5 channels per socket. It's clear, from this, the M1 is a laptop/AIO/compact-desktop chip.