The Earth-2's Laurel Lance (Katie Cassidy), will receive a backdoor pilot as one of Arrow's final ten episodes.
Season 7 of Arrow introduced a whole new storyline and new characters with a series of flashforwards that took place in Star City in 2040. At the start of the season, the flashforwards focused on Oliver's son, William (Ben Lewis). As the flashforwards progressed, William encountered older versions of existing characters, like Rene (Rick Gonzalez) and Dinah. Eventually, William met his sister, Mia, who at the time hadn't even been named yet in the main timeline. Ben and Mia's story became an important part of season 7, and by the end of the season, it intersected with Oliver and Felicity's story and confirmed Oliver's death.
It's possible "Crisis On Infinite Earths" will create a bridge between the flashforwards and the present-day storyline, and set the stage for the new series. A new behind-the-scenes photo from Legends of Tomorrow's Sara actress Caity Lotz shows McNamara with other Arrowverse actors. The words "A crisis is coming" suggests that what the actors were filming was related to "Crisis on Infinite Earths" in some way, but what does it all mean? Mia doesn't fit into "Crisis" - or at least, not what is currently known about "Crisis".
It's possible that "Crisis" will involve time travel shenanigans. If so, it would allow future characters like Mia to meet the present-day heroes. The inclusion of two more characters from different time periods, Ryan Choi (Osric Chau) and Jonah Hex (Johnathon Schaech), provides further evidence for time travel in the crossover. It's worth noting that the Jonah Hex actor is present in the same photo with McNamara.
If time travel brings Mia to the present, she could get stuck there, or choose to stay there. If so, that would align her story with the rest of the Arrowverse. Otherwise, the spinoff would be disconnected from the other shows. But with Mia in the present, crossovers would become possible.