The inevitability of Laurel taking over as the Black Canary has been hinted at since the end of the second season when Sara tossed Laurel her jacket. Following Sara's death, Laurel seems hell bent on avenging her sister and taking up her mantle. She had a brief and amazingly unsuccessful first outing that actually landed her in the hospital. And, after seeing the results of her failed attempt, Oliver told her to quit before she really gets hurt. Since it is obvious that she will not (and has not) quit training, it is safe to assume that she will reach a point where she'll become a competent fighter. At that point, she will don the black jacket (and blinding blonde wig) and begin her crime fighting career as the second incarnation of the Black Canary. Will Team Arrow accept her into their ranks? That is most likely a question for next season.