In the comics, whenever you have a Green Arrow series you know that Black Canary will show up, probably sooner rather than later. The two are as matched as Clark Kent and Lois Lane… just doesn't seem right to have one without the other. Seems it's that way on Television as well. Even though Arrow killed off Laurel Lance (Katie Cassidy) last season, she appeared in flashbacks, as an evil doppelganger and in an alien projections so far… and she was the mid-season finale twist.
Plus, we know from the trailer for the upcoming return of the show, the Laurel we see is more likely Black Siren from Earth-2 than the one we've seen for five years.
TVLine asked executive producer Marc Guggenheim about the rather spoilerish TV promo and the return:
Without commenting on the promo, which I have nothing to do with…, without commenting on the spoiler, I would say that the episode ends with laying a very specific seed, with a very specific game plan on our part that we hope to see come to fruition.
And when asked about if we'll get an actual return of the Black Canary, executive producer Wendy Mericle said:
Look, it's hard to do a show about Green Arrow without having a Black Canary. I don't know if it will happen this season or when, but… we are going to be true to origins in the DC universe, and she's always going to be there.