Not sure if it has been an idea that has been brought up before but once we get the creation kit it would be cool to see a Fallout Shelter style mod. You would be able to find an abandoned Vault, or possibly take over a preexisting one. You could build on to it in all directions. You would have to manage resource just like in the mobile game. You could build a water purifier eventually but your vault would have to be near a water source. To get food you might have to trade with wasteland traders, or with your own settlements. Another option would be interior gardens and animal farms. As far as power goes, a basic power system could be implemented using fusion cores. Where a more advanced one would use generators or solar panels on the surface. You would have to assign people to certain tasks to keep your vault running in tip top shape and as an added way to make dwellers happy you could do quests for them.

I think it could be expanded on even more to go above and beyond what the mobile game offers. If you take over an existing vault by force there is always the chance that the pre existing dwellers could revolt if they are not happy enough. For an abandoned vault, say Vault 111 for instance, you may have to repair certain systems in the vault to make it livable. You could assign certain people to help you run the vault while you are away. And as with any community there would be crime, you could decide how to deal with that. You could be a tyrannical ruler that executes criminals, or you could implement security and jailing.

Obviously there would be a lot of buildable vault parts added, I think the best way for this would be to have mostly prefab parts. Hallways, Rooms, doorways, stairwells, etc. Then you would also be able to build the various Vault themed furniture and utilities. One interesting mechanic that would probably be optional would be to actually have to pipe your water supply around your vault to bathrooms, drinking fountains, sinks,showers, etc. just like you would have to run wiring. Maybe a new wiring system would have to be implemented where so the wires aren't always showing, you could tuck them in behind wall panels and stuff.

But nothing is free, you may need caps if you intend on your vault having any contact with the wasteland, for that you could build shops that dwellers would run, or you could become business partners with various traders. In exchange for you allowing them to sell to your vault dwellers your vault would get a portion of their profits. Maybe you could even build some sort of factory in your vault where dwellers could go to earn a living and make the vault money.

Let me know what you guys think, feel free to expand on the idea. And I do apologize if this idea has already come up.