I wouldn't mind a Joker cameo, using the car footage that was cut. Maybe a scene or two before we can hear Gordon telling Batman that his 'friend' escaped from Arkham again. Show a house in Gotham, establish it earlier to be the mayor's. We hear a gunshot, and then the front door slam as we see into the dark house. We see a car pull up to the curb, and then it will cut back to the house as the windows blow out.
Now we can use the footage from the car scene, with a henchmen dubbed. 'You just killed the mayor.'
Joker: 'I'm a man of my wuurd.'
Just that keeps a stronger bond with the world that Nolan's created. Joker's a great character - you can do a lot with a little.