John Larroquette as the Joker
Okay, I know I'm probably going to get yelled at for even suggesting this. However, let's look at a few things: First, in the comic books, the Joker is supposed to be 6'7" and weigh around 190 lbs. Larroquette, when this movie came out, was a tall, slender man. Next, look at the scene where Larroquette's character confronts Bruce Willis at the party. He's covered in paint and flour with this maniacal gleam in his eye. If they had given the Joker in the 1989 movie the origin used in "The Killing Joke", I think he might have been able to pull it off.
Yippee: "For king!"
Yappee: "For country!"
Yahooie: "And, most of all, for 10¢ an hour!"