There’s an essay on the Flashback Universe on another female Joker, albeit one that predated Harley Quinn. There’s the Harlequin that Alan Scott (the original Green Lantern) eventually married. Then there’s Duela Dent, the female Joker that was to be.
It goes on saying whatever potential she had gets botched with each incarnation, where she was killed off in one ‘event’ but got brought back in the New 52.
Funny enough, similar things could said about Supergirl. Since she got firmly established as such, Supergirl was on her way to something great. Then something happened.
Characters like Kristen Wells and Starfire eat up whatever potential she’s going to have. Kristen Wells called herself ‘Superwoman’ at the expense of an adult Kara who should’ve called herself such.
Starfire is another solar powered humanoid alien and Power Girl, despite being her doppelganger, only worsens this. Then Lucy Lane jumps into the picture, further diminishing whatever growth Supergirl’s supposed to have.
Unsurprisingly all the other Supergirl reappearances get botched each time, not just with stunted or averted character development but also with rivals like Superboy, Power Girl, Starfire and the like.
It would be daring to suggest that Supergirl ended up becoming the heroic counterpart to Duela Dent in that they’re made redundant by other characters and have their potential get botched everytime.
It would also take a daring writer to turn her into something else that would really last, like a detective called The Hag to keep her from being redundant.