The press release mentions a bunch of use cases for the slots, but does anyone actually sell Apple Silicon-compatible cards yet? It's a brand new processor architecture after all. I checked the store page and it doesn't show any. (The old page would let you configure a Pro with more GPUs or afterburner cards)

Rosetta works for drivers as well (as long as they use the user-mode DriverKit interface rather than kernel extensions)

People will need to get the new Mac Pro in their hands to develop and test drivers, although any card that works in a Thunderbolt enclosure should also work when plugged in directly.