As for Shohreh Aghdashloo; as much as I love her in literally everything I see and/or hear her in, they've already established that Shaddam is a man. Other complications include male primacy seems to be a thing in Corrino succession, and as has been noted, you can't have a Bene Gesserit Emperor as it would supremely fuck with the lore of this period. The Sisterhood wouldn't make such a direct grab for power until after The Scattering.
Now, Shohreh Aghdashloo could be a good Wensicia if they go the Susan Sarandon route and fudge the ages a little, or change her to Wensicia's namesake and Shaddam's sister instead (which may just have been an Encyclopaedia thing, I forget.) Failing that (and assuming they get that far) she'd make a formidable Honored Matres.