I was playing Arkham City today, trying to get all the riddler trophies when i discoverd something interesting. now if you go into the jokers lair, and then go to the spot where you see him on life support in the beginning of the game.
Make sure to turn to the left and youll see this
And of course if we zoom in further we see this.
Now this is just a shot in the dark here, but this could be a subtle hint to the 3rd game. Since the joker died at the end and harley is now pregnant with the jokers baby maybe the sequel is harely taking over the show and raising the baby to hate batman. once again this is all specualtion, but i thought it was interesting. tell me what you think in the comments.