Can't be done due to safety rules about the maximum number of seats between aisles, the number of seating area exits ( for non pitch level areas ), and the number of concourse exits.
30,750 is fine - I'd rather have that than rattle around a 40k ground.
Maybe we could get another 500 or 1000+ seats in there somehow with a bit of ingenuity, possibly by doing something with the North Stand, but that'll surely be it.