What The Hell is Under Joker's Serving Tray?
Unless it is the head of Commissioner James Gordon. The old cop has also been a father figure to Bruce, as well as an actual father to Batgirl, and he would be well-known enough to garner the reaction that Penguin and Two-Face.
Even then, there are serious problems. No matter what is under the tray, it has to be a joke. Two-Face refers to it as such, and Joker almost sheepishly agrees. He sees himself as the jester in Batman's court, the laughing maniac able to tell the monarch what he needs to know because he frames it as a jest. Joker also clearly states that you have to play with people's expectations.
I'm going to go out on a limb here and say that since we all think there's some sort of unspeakable gruesomeness waiting underneath the silver cover, there actually isn't. My guess is that Joker will honestly serve the Bat-Family dinner, and that it will probably be one of his mutated fish from "The Laughing Fish." The ridiculous juxtaposition and cordial setting is the perfect curve ball.
After that there will probably be an elaborate and sadistic death trap because, hey, Joker. It's possible that Scott Snyder will have out-thought all of comic fandom, though, and come up with something truly unexpected. Snyder says that he aims to create a legendary Joker story, his own Killing Joke, and so far he's proven himself more than capable of living up to Alan Moore.
The thing that Moore did best, though... he ended it on a joke. A sick, demented, inappropriate for the moment joke that left Batman and Joker both laughing hysterically. The punchline is the hardest part of any bit. Let's see if Snyder can knock 'em dead