That would be quite cool. Imagine this scene (I just made it up, but it would be so awesome to see on screen, but doesn't have an actual connection to TDK):
The Scarecrow and the Joker have been in cahoots, but the Batman has foiled their plot and the Scarecrow is injured. The Joker is in a fit of rage, blaming the Scarecrow for their failure, and Crane manages to gas him. The Joker cries out in fear as the toxin takes over, turning to a corner, seemingly cowering in fear. The Scarecrow's eyes show a hint of victory and he turns to leave, but he stops as a chuckle slips from the Joker's mouth. The chuckle builds as the Joker slowly rises back up to his full height, only this time, the madness in his eyes has increased ten fold, and the smile on his face is as wide as ever. He moves over to Crane, who has already pulled off his mask in horror, and has now begun to retreat. Both move slowly, as if in a dream. Crane trips and falls, but continues to retreat, crawling towards nothing in particular. His elbow hits cold steel, and he knows he has nowhere left to hide. The Joker stops and frowns momentarily. "Well now, you don't look well at all! This stuff is guaranteed to make you smile!" His frown vanishes, replaced with a devlish smile. He reaches down and picks up Crane's toxin. "Here, try some of this," he says as he sprays Crane down with fear gas. Crane's face has slowly gone into a wide grin, but behind his eyes, there is nothing but terror. Even so, he begins to laugh horribly, a sick, painful laugh that is completely unwanted, but unable to be managed. As the Joker walks away, he says, "Havent you ever been told not to mix medecines?" They both laugh.