In the Batman Beyond comic, the Joker's legacy was given new meaning by one man. A man who would later call himself the Joker King. He claimed to "get the joke" that the one true Joker got, and through that, set a plan in motion to prove its meaning on Gotham.
With one plan, he united all the Jokerz gangs, drugged them, and turned them into walking bombs. Nearly destroying most of Gotham in the process. To stop Joker King, Terry had to unite with not only his allies, but reunite most of the Bat Family in an attempt to salvage what was left.
Terry would defeat him, but at a cost. And in an ironic twist, the body of Joker King would be used to save Bruce Wayne, who was having liver failure at the time. So Batman now had a piece of a Joker in him.