In my imaginary movie, the closest you're gonna get with the Joker is a visit to his cell or a view of his back. I prefer him to be someone people refer to during their conversations rather than taking the centre stage...therefore really there're only two villains...I think the Joker should be retired the way the Yankees retire Babe Ruth Jersey, at least in the Nolan franchise. And you're right it's a fitting tribute for Heath too.I could totally see something like that.
Now, here's what I wanna know... if the Joker's around, will he be an unseen menace, like Frogboy suggested, or will he be recast? I feel the former works better, as a tribute to Heath... and if the Penguin and Riddler are also there, I feel it would be pretty cluttered with 3 villains (Since the others had around 2... Scarecrow was mentioned... don't know if that was really him)
I think the Riddler might become an extended Harvey Dent as well cuz he's the good bad guy so you can create some sympathy for him like they did for Harvey...but Two-Face only exploded for like 10min. and the Riddler can explode for the whole movie