I highly doubt Apple will make drivers available for 3rd party video cards. It opens up a whole bag of worms for Apple that they were able to eliminate when they migrated to Apple Silicon. Assuming the price will be similar to current Mac Pros, professionals will be the target audience, not gamers. Maybe they'll have some sort of Super Afterburner II cards available...
The only real uses for the PCI-e slots that I can think of are for:
40GB NICs?
1000GB NICs?
Storage.
A/V I/O.
Btw, there's no chance in hell it will be Intel based. Zero. Nada. Zilch.