In Subway Town, during the last story missions, a room is being raided by the Authority at the 2nd floor. Also at later part of the story, down at the first floor where you talked to Foreman Jones for the "Mutant Expansion" (your first mission from Stone) there is now a block with the sign that says "Subway Town Phase II Expansion Coming Soon!".
Here's my theory on a possibly big chunk of dlc: The room that was raided by the Authority, there were two men inside that you had opportunity to eavesdrop on before the raid. My guess is they are characters for a co-op that's gonna be set in the Subway Towb Phase II Expansion.
http://i233.photobucket.com/albums/ee313/anno234/34a5033a48a2__1319389552000.jpg
http://i233.photobucket.com/albums/ee313/anno234/4964755088bc__1318579928000.jpg
Have you read up on the "hidden buttons" and hidden "sewer script" where you have to find these randomly scatter pixels to click on, which throws some phantom button, then opens some hidden elevator... they are scattered in at least 5 of the sewers I think it is... the dev/sig/trophy room, the gold posts achievement, doom room, wolfenstein room, quake room,...
on...
and on...
and on...
This game has more easter eggs than missions - don't hold your breath, id's 'messages' are MUCH more hidden.