The Sole Survivor emerges from Vault 111 in search of their son and to seek revenge upon the one who murdered their loved one.
But there was another to emerge. The Survivor. Confused to see that the Vault is in ruins they do see some signs of recent activity. 1 empty pod. 1 pod open with a bullet hole. What happened here?
Exiting the vault and stepping into Sanctuary the people kind people there tell of the one before you now the commander of the Minutemen.
A nugget named PressedHam Mc'Gravy offers you to be a Sargent. You promptly refuse and begin your journey.
Jokes aside, this DLC would be for those that want to be their own character and not Nate/Nora in search of Ben. You would be able to go off and find what happened in the vault and possible eventually meet the SS.
Probably to much dialogue and to many loop holes, but whatever. Maybe the SS is stuck at Sanctuary listening to Mc'Gravy tell them about the settlers and Lady Hates the world and Dudly Wiener hammer on that building all day.
In short let us roleplay as a character WE create. Some of us like that kind of thing.