Build a beautiful online world and you can be sure intrepid explorers will find a way to escape it. In this instance, efforts by members of the Division community have revealed locations which might hint to future DLC.
Chaz leveraged a couple of wall glitches, including a convenient vault and liberal use of the Ballistic Shield, to access incomplete fortified areas and discover a highway heading by Central Park. The park's not quite as pretty as the real thing, but if any New York landmark becomes the setting for a high-stakes post-apocalyptic shootout, I bet it'll be this one.
Or it could be the USS Intrepid uncovered by Redditor Orut-9. I feel we can be safe in assuming that an aircraft carrier is not a placeholder asset.
These might just be relics of abandoned ideas—World of Warcraft's Hyjal and Shatterspear Village went unused until its third expansion—but there's a lot of potential to be seen when you go off the grid.