Do I think The Joker used to be Robin in the DC Extended Universe? Nope. Do I think it's possible?
Yeah, actually. Dick Grayson taking over the mantle is something which happened in The Dark Knight Returns sequel, The Dark Knight Strikes Back. Jason Todd meanwhile took over one of The Joker's personas (The Red Hood) when he returned from the dead, so there's precedent for this happening in the comics. Even so, this would be a move sure to upset an awful lot of comic fans!
Personally, I would hate to see Jared Leto's Clown Prince of Crime be a former Robin, but it could happen. Ben Affleck's Batman has been operating in Gotham City for two decades, leaving plenty of time for the original Joker to have died and for the once Boy Wonder to decide to inherit the mantle in what would be a truly sick joke at the expense of his former mentor. So, with speculation going into overdrive as people claim to be finding evidence which links the two characters, do you think they're one and the same? If so, just how upset would you be if Warner Bros. did this?
Here's your chance to let us know, so cast your vote and be sure to sound off in the usual place.