Like I've said before, I sincerely want a shot of Joker, in a bullet-resistant glass cell. A bed and toilet on opposite sides of the cell, and a plastic stool in the center. He's on it, wrapped up completely from neck to waist in an "overkill" straightjacket(double the buckles and straps, maybe even a few chains). There are overhead lights, so his face is partially hidden in a light shadow. His leg is bouncing up and down and he's just giggling and twitching. At Joker's back, where his bed is, on the other side of the glass, is a computer deck with monitors and keyboards, the monitors are showing what the cameras, positioned at almost every angle around Joker's cell, are looking at. Every angle of the Joker is watched by a guard. But, since it IS Gotham, every night at around the same time, the guard goes to get a soda or coffee or whatever. He turns on a security alarm system and leaves. And the position Joker is in isn't random. He's facing that direction for a reason. Out of a shadowy part of the room, Batman steps forward. He does this every single night at the end of his patrols, to make sure Joker is where he belongs. And all Joker does is just stare. He stares at where Batman will be, is, and then, for a short time after, where he was. Twitching and giggling to himself the entire time. Like someone who just heard a really funny joke and was dying to share it with someone.
And that's really all I want Joker to do in this movie since Heath is gone. Simple, incredibly effective, and it gives us a bit more Joker without it stepping on Heath's performance so soon after it blew everyone's mind.
Which is why I nominate Joseph Gordon-Levitt. He has a good, Joker-like build, VERY similar facial features to Heath, so the prosthetics and make-up and such can remain very much the same. And he's a DAMN good actor.
Hell, we could even show a shot at some point of Joker staring at the spot where Batman stands every night, and here over the intercom from the lazy guard, "Hey clown, your hot, blonde shrink is here again. Fourth session in a week. Must be making some PROGRESS." And Joker looks over his shoulder and just winks(or something).
That'd be perfect, and give EVERYONE what they want.