Since the Legends of Tomorrow protagonists are riding around in a time machine, it’s conceivable that they could somehow run into a past version of Laurel.
Sara Lance has sworn off trying to change the timeline to save her sister’s life, but circumstances may force the heroes to interact with Laurel to preserve the space-time continuum or something along those lines.
Katie Cassidy was a series regular as the Earth-1 Laurel Lance on Arrow for its first four seasons, where viewers watched her rise to becoming both Star City’s assistant district attorney and the Black Canary.
However, that’s not the only possibility on the table.