So in the second mission if you get access to the sewers to help restart the water pumps for the Hatters, you have a chance to talk to Delilah (or her statue). She says a bunch of stuff but one particular sentence stood out for me. She says "Four of us chosen, but history will remember only one."
Obviously I assume she's talking about the Outsider bestowing supernatural powers to four specific individuals. Now, I only know of three individuals with the Outsider's blessing: Corvo, Daud and Delilah. Who is this fourth person? Are we to assume it's Granny Rags by any chance?
I haven't finished the DLC by the way but I doubt they'll reveal the fourth person by the end of it. As far as we know, this is the last planned DLC for Dishonored so I very much doubt it's somebody new who we haven't been introduced to yet.