I like the settlement system but I would like to see more options and see it expanded a lot more than the current system. Surely this idea would allow a bit more freedom for us settlement builders...
I think I understand what you're saying. You'd like to see Settlements that are built in interior cells that behave as exteriors. I wonder if someone could convert Hangman's Alley into this kind of cell. Based on how it's situated, it'd be the perfect place to try something like this.
There definitely aren't any of these in game and it would definitely help build some pretty sweet things.
I'd also like so see the few interiors that are connected to settlements (I can think of two right off, but shall not name them due to spoilers) count as part of those settlements. Have NPC's interact with them and edit their spaces.
I was thinking this would be a great idea for a Vault Settlement similar to fallout shelter. it would be cool to be able to scrap all the items in the vault and build it from the ground up and having vault themed items to place.