For me, her inclusion in BTAS gave us something iconically new, in an industry where it's incredibly difficult to come up with new, sustainable characters, and turned the gangster moll stock character and trope on its ear. Let alone it being incredibly challenging to add something new that resonates to the Batman mythos - one of the largest and developed ongoing dramas in all of speculative fiction.
The only other non-classic character that I can see became as sweepingly big as Harley (before her) is Wolverine.
The BTAS version of the character just works and is a great juxtaposition of the "hero and girlfriend" trope too. I'm actually surprised we haven't seen more attempts at developing villains's significant others because of Harley. Not to copy Harley, but to explore that dynamic.
My favorite quote about Harley that sums her up succinctly and just "hums," was by Dini (I think) as she was being developed:
"Q: What's scarier than the Joker?"
"A: The woman who loves him."
So much to unpack in that and play with in terms of themes, gender roles, stereotypes, etc., as well as the Batman character mythos hallmark of mental health challenges.