Something I've been thinking about. In Arkham Knight is the Joker just Batman hallucinating, or is it really the Joker's ghost? Yes, a lot of evidence points to the former, but the Joker shows things that Batman knew about but didn't actually see firsthand, like Barbara being crippled. Not to mention the fact that when the Arkham Knight reveals himself as Jason Todd, the Joker admitted to Batman that he lied about killing Jason.
Oh, and let's not forget that the Joker actually hijacks Batman's body during the endgame; could a hallucination do that?