Possibly an obvious one given how many interviews Rocksteady have given where they insisted time and again he's definitely, definitely dead, but it's not until the Arkham Knight tie-in comic where Joker finally has gets his autopsy and is pronounced fully, actually, unequivocally gone from this world. What's that? Yes, he does have a USB drive under his skin that emits laughing gas and then appears on TV to... no, no, he's definitely, absolutely 100% dead. Even if he isn't, the events of the comic show the Arkham Knight running amok killing a certain villain at the same time Joker's being carried off by the GCPD, so even if he IS still alive - which he most definitely isn't - it's not the Clown Prince under that mask. Probably.