Batman: "I will make an exception in your case."
That one line would stir shit up proper, add an unpredictable element (for a while) to what has become THE most predictable villain, AND breathe some life into the Batman character. Every time Joker showed his face (or pulled it out of his pocket, or whatever) he'd be taking his life into his hands. They wouldn't even have to actually go through with it, just the notion that Batman has had his fill of Joker and WOULD go through with it would be enough.
or, how about an occasional story where the Joker just does some wacky shit and doesn't actually kill anyone at all. Because right now, it goes a little bit like this:
Joker shows up out of nowhere after having been presumed dead, or whatever
Kills a henchman who makes a slightly innocent remark which set him off because WOOOOOOOHOOOOO he's crazy
Kills a whole lot of people just to get Batman's attention
Batman shows up, beats the shit out of him, stops himself at the last second because I CAN'T EVER CROSS THAT LINE MOMMY AND DADDY
Joker word balloon with shaky font comes out of Arkham Asylum window OMG HE'LL BE BACK
rinse, repeat, zzzzzzzzzzzzzz