I get a lot of leads and inspiration from my comrades in the Crossover Forum over in Facebook, especially from Robert Wronski, Jr. Recently he posted this:
In the Angel episode "Smile Time", one of the lab workers at Wolfram and Hart suspects the Joker is behind the large amount of children dying with big smiles on their faces.
The Joker seems to be mentioned as if he was a real person, which in Toobworld he was. The Clown Prince Of Crime was visually active (and what a visual!) in Gotham City during the 1960's. Before that, he had been a schizophrenic operative named Victor Gervais working for THRUSH in New York City. But then, after the post-traumatic stress of being shot, he was a double agent in Washington, D.C. using the alias of Kinsey Krispen.
After he nearly drowned in a vat of the pasty white liquid which was used in making his statuary, his mind finally snapped and he became the arch-criminal The Joker.
The Joker, like the actor who portrayed him in Earth Prime (Cesar Romero), died at some point around 1994. But that was Gervais/Krispen. "The Joker" is a nom de delit that could have been assumed by any other criminal.
In fact, 1994 on the Toobworld timeline could mean that the Joker might have been murdered by the Trickster, who either wanted the sobriquet for himself, or was just following the tradition of the young gun supplanting the legend.
So The Joker could still be out there, with the same hideous facial distortion and coloring, but with some other splainin as to his origins than the one I supplied for Gervais/Krispen. And since The Joker's real identity was never supplied (within the TV series 'Batman'), then anybody else could now be accepted in Earth Prime-Time as the heir apparent to the Clown Prince.