Going to the commonwealth would be a great idea. Imagine Fallout: Operation London, Big Ben in ruins.
http://fallout.wikia.com/wiki/Commonwealth is what was once the Commonwealth of Massachusetts, and has nothing to do with Europe.
At any rate, no DLC could do the Commonwealth justice, it has to have it's own game imo.
DLC Name: Hail to the Commonwealth
Description: Based around the Commonwealth faction far to the north-east, scout scientists of theirs have unearthed the lost installation 'Area 51'. In scouring the facility, however, they've not had the least bit of luck since; with androids going rogue, and teams going missing deep within the find. The player finds their way to Area 51 by way of an alarm system that he picks up as a radio signal, with increasing severity and clarity the closer they get to the source, making the first quest to be simply finding the place. Once there, they have the opportunity to help the scientists recover their lost research teams deep within the facility, no crawling with blind underground dwelling mutants of an unknown source. Various sidequests would also detail an extended Replicated Man quest in which the player is charged with identifying rogue Androids through speech options and either choosing to help them, or take them down for a bounty. Inspirations would be Blade Runner, Alien, and HP Lovecraft among others.
Why on earth would scientists from the Commonwealth be down in Nevada?
We are given a book that we have to take across country, but we are stricken blind.
I can see the reviews for it now:
"The graphics are worse than Pong. The entire DLC is literally nothing but a black screen."
"There's a huge world to explore. Or at least I think there is. I may have just been running into a wall the whole time."
Also... that's ripping off The Book of Eli quite a bit.