Unfortunately, it has already been confirmed that Katrina Law is only on board for one episode of Arrow this year, so unless something changed since the announcement, the two aren’t on a path for a reunion.
The observer will bring Sara’s old flame Nyssa al Ghul to Star City and result in an event that the Arrow team teased as “unforgettable.”
Sara may also have some role to play in another storyline, as Arrow recently revealed Thea Queen is being observed by someone who knows she’s the heir of Malcolm Merlyn’s Ra’s al Ghul.
That said, whatever drama the situation creates could draw Sara back into town, but that speculation is mostly rooted in her past with the League of Assassins.