And this seems like a pretty big duh, of course.
The Apple Silicon MacPro will have to set itself apart with impressive CPU and GPU power as you are not going to be able to add either of these and I am even doubtful of PCI support. And I doubt there will ever be external GPU support through Thunderbolt on M1 Macs either, unless apple ever figures this out for their own hardware, but likely they would just like you to buy a new machine.
I am tending to doubt their will even be additional bays for RAM or Hard drives, though with a likely dual or quad M1 MAX, it will certainly be larger than a Mac Mini and need more cooling, but I doubt it will need to be too much bigger.
I would like to see more Thunderbolt ports though. The 4 Thunderbolt Ports on my iMac Pro are kind of a joke, and the MacPro has more and the video cards also add 4 additional ports, so 8 or more thunderbolt ports would be awesome (and 12 would be better, or at least 8 thunderbolt and 4 USB.
And maybe the ability to have 2 raided hard drives for video, but with the prices Apple charge for SSD hard drives, that would cost a fortune, but it would be great for video editing.
Any which way it will be the raw processing power of the chips and the included ProRES accelerators that really makes this machine rock and roll for video editors.