I'd like a DLC that adds content to the main game. This would probably be hard to market, so I doubt it will happen, but wouldn't it be cool?
Open up some of those boarded up buildings.
Add some more enemy spawns, or at least mix 'em up some.
Add a couple unique weapons or sets of clothing/armor.
Add some longer dungeons (I know, I know, but what do you call them?)
Maybe try to reflect some changes that have occurred. Looters in Nipton. Caravans on the 15. Stuff like that.
tl:dr -- add some surprises.