Arrow's Katie Cassidy Probably Isn't Gone for Good
The CW's DC TV Universe is a place where the fantastical and impossible happens pretty much daily now, and like the comic books that spawned these shows, death seems to be temporary state of being, at worst.
Case in point are the Lance sisters on Arrow, who have died (in Sarah's case, multiple times), only to resurrected again. Season 4 saw Laurel Lance lose her life at the hands of Damien Darhk, but she's not been all that absent, since then. Actress Katie Cassidy appeared as Laurel in fantasy sequence during Arrow's 100th episode, and more recently she played Laurel's Earth-2 doppleganger Black Siren, who sold Laurel's resurrection to Team Arrow in order to infiltrate them on Prometheus's behalf.
The producers of Arrow and the larger Flarrowverse have repeatedly hinted that there will be more of Laurel Lance/Black Canary to come - and now we have a better ideas of how that might be done.

During a Q&A event with Arrow producer Marc Guggenheim, Comicbook.com was among the press present, as Guggenheim described how Laurel may reappear in the Flarrowverse.
According to the show's EP, "We have an idea for how to see her again, but we haven't made a deal with her yet. She's not a series regular anymore, so we have to make a contract with her and she's got to be available, and we haven't had those conversations. But we do have an idea. We know exactly what we do want to do."
At this point, there are literally several dimensions and innumerable timelines that Laurel Lance could be plucked from. The Flash and its free run with multiverse theory has made it all possible, so narratively, there's no problem. Behind the scenes, Cassidy has always been generous with fan service; when she wants to play in the Flarrowverse sandbox again (or if producers need her), she'll stop in for a visit - which most Arrow fans would probably agree is enough.
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