One of my favorite FO3 mods was the DC Interiors project.
There are lots of intact, boarded up buildings and stores that already have signs and stuff up that you can't go in (many in Freeside, and many more outside of Vegas).
Would be great if someone or some people decided to make an Interiors project mod for this game as well.
Not just a bunch of empty rooms full of swag, too. I mean, those would be easiest, certainly, but adding a little bit of content would be welcome as well.
New shops, maybe. Etc.