I would love a DLC that gave you the experience of two different people living in different vaults. It would start with them growing up and you see through their eyes. You would make choices that have an impact on what you later find and read in the vault as you enter them with your Fallout 4 character. They have to keep both stories short but intense. And with all the experiments Vault-Tex did I would also love it if one of them featured a lot of scare moments, and you had no weapons and had to survive. I know some might hate that and think it's way to close to Alien: Isolation, but it's not like that game is the first to use the theme.
It would give you more to do because of replay value. For those that don't want to play as these two people they could just enter the vaults and get the default loot and stuff to read and see.
Edit: It would give us all a chance to live through the horrible experiences we have only been able to read about when you entered the vaults long after they happened. At least for me it's one of those things that is missing; to really be there.