I have never purchased a CE, but find myself wanting one for this game.
I would welcome a Soundtrack and Quality statue.
A physical rule book and exclusive spell would be great. Perhaps something unique for each class, so regardless of what you play, you can have something to use.
I don't know if it is even being looked at, but if I recall, there was an option to make custom campaigns in DOS2... if there is the same possibility in BG3 I am sure that it would be a thought to have a DM screen along with the ability to create custom campaigns in the Forgotten Realms landscape.
I am sure that there are other great ideas for a CE.