In the announcement for Far Harbor:
A new case from Valentine’s Detective Agency leads you on a search for a young woman and a secret colony of synths. Travel off the coast of Maine to the mysterious island of Far Harbor, where higher levels of radiation have created a more feral world. Navigate through the growing conflict between the synths, the Children of Atom, and the local townspeople. Will you work towards bringing peace to Far Harbor, and at what cost? Far Harbor features the largest landmass for an add-on that we’ve ever created, filled with new faction quests, settlements, lethal creatures and dungeons. Become more powerful with new, higher-level armor and weapons. The choices are all yours.
I know it could very well be a random young woman, but being the rabid hound for speculation that I am I was curious to see if we might have any guesses. I personally just had the thought, what if the young woman you are looking for is a synth version of your wife? I mean, we already have synth Shaun, so it doesn't seem too entirely out of the possibility that your wife could have been made into a synth. I mean if you stop and think about it for a moment, they can use any pretense to get your PC to do anything, but I'd like to think that the main story of the DLC will be a compelling reason to get out of the Commonwealth, besides Nick Valentine coming up to you and saying, hey I got a case for a missing woman, want to team up?
Big plot hole that I just realized; you can obviously choose to be a woman and so you would be looking for a young missing man in that case, however maybe in the case of the press release, they just decided to default with a male sole survivor? Especially since it is the male sole survivor in the dlc art pictured.