In one of the 'Arrow' episodes this year, the Batman villain and Joker sidekick Harley Quinn may have turned up... but in voice only.
Tara Strong gives voice to the character in a different fictional platform (video games), but was listed in the episode's end credits only as "Deranged Squad Female." That could apply to Harley Quinn, but that could also describe any number of characters in the DC Universe. It could also be the televersion of a woman I dated back in 1998!
If she is Harley Quinn, it wouldn't be the first time she appeared in the TV Universe. The Tooniverse has several incarnations of her, O'Bviously, with all the different animated series featuring the Batman, Superman, and even Static Shock. But there was also her appearance in Evil Toobworld in episodes of 'Birds Of Prey' and in the Claymation/Stop Motion world of 'Robot Chicken'.
Earth Prime-Time came close to having a Harlequin, if not a Harley Quinn (aka Harleen Quinzel), back in 1990: Zoey Clark tagged along with the psychotic Trickster, calling herself "Prank". She may have gone on to change her alias to Harlequin in an attempt to distance herself from the Trickster after he kicked her out of his car at high speed. But she still would have been Zoey and not Harley.
Will Tara Strong eventually show up on 'Arrow' as Harley Quinn? Executive Producer Marc Guggenheim had this to say about her eventual return:
"I want to manage expectations with respect to Harley Quinn. She was always intended to be an 'Easter egg'. I don’t want people to go in with incorrect expectations and walk away from Episode 16 disappointed."
Since each character's rights have to be negotiated separately with D.C., it may prove unlikely. They may be saving her for their next comic book-based series, 'Gotham'.
Episodes of 'Arrow' jump back and forth between dimensions (usually depending upon appearances of Barry Allen.) So overall it may prove inconsequential to the Toobworld Dynamic.