The characters arriving are QoD and Rumple. The characters leaving are Emma and Regina, who may be leaving town in search of the author based on an earlier spoiler last year in which A&E said the search for the author would take them out of town. Since the title of the episode is Lily, I’m now beginning to suspect she’s the author and/or sorceress.
One of the other theories is that she’s the offspring of the author/or sorcerer. If that were the case, maybe Emma and Regina already found them, but they asked them to go find their child (Lily) for them outside of Storybrooke? That sounds like it would be somewhat of a re-hash of the whole “Search for Baelfire arc”, except Emma is teamed up with Regina instead of Rumple and Henry.
Since they rehash a lot of their stories as of late and most of the arcs are formulaic right now, it wouldn’t surprise me at all if they rehas the Search for Bae