Ok I cannot help myself so i'm going to say it. Fallout 4: Black Flag.
Yeah I know weird.
I loved Assassins creed 4 black flag.
I'd love to see similar in fallout 4.
How about dlc where an abandoned pre war ship of some kind runs aground and is mostly intact. At some point in fallout 4 you are connected to a faction or two, so you can get together with your allies to survey the vessel and refit it for use.
Then you have a totally new exploration mechanic to utilise.
With use of the Building construction system the final look and abilities of each "Finished" vessel will be different each time.
With exploration along the east coast the possibilities are endless.