Underwater. Fully functional, but they can not come to the surface.

Cave. Why no one didn't escaped into some deep cave?

Billionaire vault. This was mentioned in the game. Paranoid billionaire has made his private shelter. But no one did not survive long enough to have descendants. It would be nice to have: Gatestown, Trumpcity, Zuckerbergville...

Space Station. Also mentioned in the game. What happened to them? Are they still alive? Do they even exist?

Secret vault. Vault without number. Made by the Vault-tec, without knowledge of government.

Aircraft carrier. Ship's captain decided to remain at sea, with the 6000 crew members.

Cryonics Institute. Totally not related to the war, there was deep freeze experiment . Because of the war, this experiment was disrupted. Now Institute ran out of energy and everyone wake up in a new world.

Sounds like some really cool ideas there. Would love to see them really fleshed out and appear in a game, if not F4 maybe the next one

I mean, for the record, Vault 87 counts as a cave vault, since it was built into Lamplight Caverns. Vault 111 most likely involves cryogenics. It looks like there's going to be a lot of stuff going on in the water (otherwise there wouldn't be a perk that lets you breathe underwater), so I wouldn't rule out an underwater vault.

To be fair the first side quests in oblivion gave a ring that let you breath underwater and there wasn't much to do under the sea... :/

To the OP: I love your ideas! If they did any of those I would be thrilled. My favorite is the billionaire vault... Because there had to be rich people who had the money to buy personal vaults.

I did a search, there was one of these in Fallout BoS.

The underwater vault is definitely feasible with all the water we have seen in map previews.

Well, the way I see it, whereas enchanted items that let you breathe underwater don't actually require any deliberate investment, a perk does. I don't think they would have put a perk in that lets you breathe underwater unless there's a lot going on underwater. Then again, now I'm wondering if they're going to have a rebreather like in New Vegas. It would be rather unrealistic and disappointing if there are human gills but nobody has managed to come up with an oxygen tank or CO2 scrubber.

I wouldnt be surprised if the radiation suit/space suit would also work underwater, after all it uses oxygen tanks

Good ideas. I really like the idea of a sunken ship at the darkest bottom of the sea, with rumours of hauntings or creepy stuff.

Maybe even a creepy (haunted) vault?

I would like to see a:

Vault full of infants raised by Mr. Handy to see how humans can survive into advlthood without human interaction.

Vault of religious zealots whom think it was God who destroyed the world.

Vault that was waited on hand and foot by Mr. Handy and never had to work.

Vault that was only allowed to eat meat if it was human.

^^ Now, that I was talking about. Give us a completely new ally or enemy.

Is not point that to see just a cave or a ship. It should be a completely new society.

Cave town, there are thousands of people who have never been exposed to radiation. It would be a holy grail, for the super mutants who are still follow path of master.

Space Station, they are forced to develop technology at any cost, just to survive. Even enclaves can not compared with them.

I'd like to see an Experimental Vault where the experiment was foiled either by the Vault Dwellers or maybe a contractor correcting what he saw as a design flaw without ever telling anyone.

Say for example Vault 53, most of the equipment was designed to break down every few months. While repairable, the breakdowns were intended to stress the inhabitants. Well what if the contractor noticed the design flaw in the equipment fixed them and told no one about it.

The Overseer who is typically aware of the experiment slowly goes insane because it never commences.

A vault filled with nothing but convicted criminals.

Australia?

Most of them where political prisoners or people some wanted to get rid of.

Didn't the New Canaanites come from a vault? From what I remember they were Mormons who believed that god ended the world. They may also have been a part of an experiement where their clothes disintigrated and were unable to be replaced anymore.

Vault 111 is a cryonics vault i think