The plot of the young, orphaned child showing up at the characters' doorstep is a fairly common sci-fi plot, but I'd say "The Runaway" is a step forward. I mean, it's not as common as angry birds or amnesia. Maybe the writers will get down to ancient artifacts by the end of the season.
Through the story of Leah showing up, the audience learns a couple new pieces of informations: Mira kept a mysterious container buried and wanted it back, and she made a deal with the people in the future so she can get her girl back. Now, none of this information is particularly useful since we still essentially know nothing, and the dialogue is shrouded in Mystery, but it's a start.