I'd just prefer some more living space for the figures, as I like to roleplay. Also that room with the Knight seemed useless, it should have been a bathroom like the reviewer mentioned.
A secret room would have been nice, like the room of Requirement. But that could be a stand alone set.
Oh and Malfoy should have been included, he's the only figure that really should be in this set as well. Especially since there is a Slytherin common room.
But alas perhaps I'm begging for too much, I don't own the previous Harry Potter castles and from looking at them this one is quite nice.