The show runners on The CW's "Arrow" are fairly free to use what elements of DC Comics lore they like for their series, though some parts have been declared off limits due to the studio wanting to use them for their upcoming Cinematic Universe.
One element that got teased last season was The Joker's offsider Harley Quinn. The character appeared only as a shadow and spoke one line in an episode focusing on the Suicide Squad. Unfortunately she won't be back according to producer Andrew Kreisberg who tells CBM:
"There are very few things we've asked DC Comics for that we haven't gotten. That [Quinn's appearance] was something you asked for, and what you saw is a compromise that Geoff Johns and I came up with. To even get that much was a thrill for us.
I know that they may have other plans for her, but it was certainly an Easter egg that thrilled us. But the Suicide Squad will definitely be back. Whether we see the 'deranged female killer' we saw before, we'll have to wait and see."
Kreisberg wouldn't elaborate on those "other plans" for the character, but considering the show is able to use a major Batman villain like Ra's al Ghul, it indicates someone at Warners/DC has something in mind.