As Black Canary's mother. Who was also the Black Canary, come to think of it. Though I doubt that'll be true on the TV show. http://insidetv.ew.com/2013/01/22/arrow-alex-kingston/
The more things that both of them do, the less likely a Captain Jack/River Song spinoff will happen, and that makes me cranky.
But maybe the Moff will see them together and start thinking about finally having the Whoniverse's two most flirtatious characters meet. After all, Jack has two missing years that haven't been explored and there's also the similarity in their choice of weaponry. (Each uses a sonic blaster.)