Obviously, a book cover needs to provide structure and protection for the book. But what is the purpose of the design, and what does the design have to say about the story within it? The artwork on the cover could be anything, really.
The cover could have people on it, such as the protagonist, or an abstract figure, or crowd of people who belong in the world of the story. It could have an object that relates to the title or the storyline. It could convey a sense of the story through typography only, choosing a typeface and way of setting the type that conveys the time period or mood of the story. It could depict an environment or building from the story. Or it could have an abstract image or pattern that conveys the mood and general atmosphere of the book, or the cultural setting and traditions.
With so many possibilities out there, the question that we sometimes forget to ask is not what can a book cover be, but what should it be?