What I'd like to see is a sort of Easter Egg.... our hero finding traces of (brace yourselfs) The Courier.
Well basically Loansome road gave us a bit of info about The Courier's past..... at least thats what i'd like to say.
Truth is that before Loansome Road , I only thought the courier was your average Joe that got cought up in a big mess , which, after much determination and luck he was able to solve , as skill came along the way.
But after Loansome Road , The Courier seemed to be quite the legend, a skilled adventurer that could overcome a lot of trials.
Thats why I think that a Fallout 4 DLC should be about our heroe following clues about The Courier's past before New Vegas and after so that he could find what finally happened to him.
Because after NV ended our hero seems to have hit the road again towards new adventures, so he could have gotten to the Commonwealth, he might have also been trough the areas of the other Fallout series.
The reason why I wish for this is because, unlike the other series, I never got to know what happened to the courier in the end, since he seems to have just left and dissapeared at the end.
Also if you would be able to make saved games from the previous fallout games (fallout NV at least) have an impact on the DLC's lore as a bonus, that could also inspire those who are new to the series, to give the previous fallout games a try.