It occurred to me that a nice DLC would be one that let the player start a new character who was not the Sole Survivor. After finishing the main quest, many players just want to wander and stuff. Why not a DLC that lets them do that from the start? No need for an in-game backstory, just create a character and place him at the entrance of the vault. Then, go do yer thing, playing the character you want to play.
There would be details to work out, of course.