Laurel Lance is back in Star City, thanks to time travel and a save from her time traveling sister. At least, that's what she tells Oliver at the beginning of Arrow's winter premiere, "Who Are You?" Thankfully, a suspicious Felicity does a little digging and realizes the Laurel standing in the bunker isn't their Laurel brought back to life, she's Earth-2 Laurel, who escaped from Central City courtesy of Prometheus. Even though Laurel didn't turn out to be the woman Oliver thought she was, Black Canary is back on Arrow, just not how you remember her.
To be clear, the Laurel that returned at the end of Arrow's mid-season finale was not the real Laurel, it was Black Siren, her evil Earth-2 doppleganger. Turns out, Prometheus broke her out of prison and wanted her to cozy up to Oliver and the rest of Team Arrow, but Felicity finding out her true identity put that plan to rest. Once they knew who she really was, she fed them a sob story about how Prometheus had threatened to kill her, and that's why she did what she did. (To be fair, he did threaten her life.) Oliver wanted to give her the benefit of the doubt, but Felicity did not. In the end, Felicity went over Oliver's head to call the shots, something that resulted in them almost stopping Prometheus, but failing at the last moment, when Oliver chose to save Felicity and capture Black Siren instead.
Oliver hasn't given up on giving Black Siren a second chance, but more than that Laurel's brief return into his life made him remember of a promise he made to the real Laurel. When she was dying, he promised to train another Black Canary to take up her legacy, and, while he was resistant at first, he's ready. Now all he has to do is find her.
Lucky for him, there seems to be just the right woman in Hub City. The woman, an unnamed brunette, is a meta who appears to have the same powerful Canary scream as Black Siren. As for who she is or whether she'll even make it to Team Arrow, we'll just have to wait and see.