Nope, the official BMW top case, on the official BMW luggage rack, will NOT require any sort of plate.
Take another look at that big picture in post #6 here, on page one. This shows the new case on the new rack, and while you can't see what's underneath the case, there's nothing to see there. Go ahead, look at that pic now, post #6 ... I'll wait ...
Now, here's a screen grab I just took off of the BMW web page -- the Build Your Own page, with a 360° rotation function. I rotated the pic there, and then annotated it:
View attachment 5463
The arrows point to prongs (or whatever you want to call them) that the official top case slips into, and then locks into, when mounted.
This is how the Shad case works, too, except that the prongs will be on the Shad base plate, which base plate gets bolted ON TOP OF the BMW rack. (And is how a Givi case works with a Givi base plate, etc. -- I had Givi cases on two big Burgman scooters, for instance).
So, if you go completely with a BMW setup -- new case and new luggage rack -- there will NOT be any base plate: what you see here is all there is. That's how the old 30L BMW case worked on the BMW luggage rack on my '22 and '23 models, and as I can see the same sort of prongs here, I'm sure that's how it will work with the revised luggage rack and new bigger case.
Follow me?
Of course, with the USB-C and central locking, there will also be some sort of wiring connector to the case, which you'll have to disconnect when you removed the case. I had the same sort of situation with my '12 Victory Cross Country Tour: it came with a huge top case from the factory, and had a built-in LED taillight and brake light; if you wanted to take that top case off you had to remove a couple of bolts, and there was a twist-to-disconnect connector by the passenger seat.
This all make more sense now?