Posted in another topic, but figured I'd just make this its own topic....
Vault-Tec Headquarters DLC:
I think this would be amazing for not just lore, but imagine being able to see complete lists of all the vaults across the country. Every vault is pretty much a science/social experiment, why not get a view from the guys who actually ran things. It wouldn't be just a single building, but an entire campus with its own underground tunnels and separate districts. You could have it being raided by a company size BoS element, but due to the automatic defenses, they lost all communications with the commonwealth. Inside it would have everything from heavy turrets defending everything, to a Vault-Tec faction and even have those underground tunnels filled with ghouls.
You could even have the community come up with the different experiments in each vault. What would be all of your guys pitches for a vault experiment?