Say Joker did die for good and soon after all of the villains in Gotham either got locked up for good, killed, retired, or simply and for a few years it was peaceful then a new generation of villains popped up each just as deranged if not more so then the Joker. Then while fighting them all Batman discovers Joker had a hand in creating them either as their mentor, inspiration or the guy that made them go insane. Ultimately these new villains drag Gotham back down into darkness destroying everything Batman worked for and Joker reveals via a pre recording that he planned it all even his death and made it his legacy to destroy Batman's hopes and dreams. Can you see Joker having that big of an impact?
I personally think it would suit him I mean what better way to screw with Batman then to let him taste a peaceful Gotham and then from beyond the grave rip it away from him leaving Batman no way to take revenge. Besides Harley can't be the only lunatic that would practically worship Joker and want to carry on his legacy of madness.