I actually thought that the toxin released in Gotham along with the escaping Arcam Asylum inhabitants was forshadowing in order to set the scene for the Joker. It would have made an excellent reason for the appearance of a diabolical insane criminal.
If you do a search, you can find the entire "The Man who Laughed" on youtube. which was the original inspiration for Joker. He looks exactly like the original Joker, down to the bone structure, hair, nose, everything.