Back in September Naughty Dog published a post on PS Blog talking about how they were planning three pieces of DLC for the title and it looks like the third of those will be with us very soon. You can see the whole post here. The info we were given then was:
"We’ve been hard at work making the DLC content for The Last of Us – and I know that all our Season Pass holders are eagerly waiting for any updates or news. Today, we’re revealing the first of three Season Pass DLC drops"
Later in the article, Naughty Dog mentions: "Our third Season Pass DLC is multiplayer focused and will arrive in the first half of 2014." This seems pretty reasonable, however, this tweet from Bruce Staley, Game Director at Naughty Dog suggests something different:
Played some of #TheLastOfUs's new SP mode today & got owned... and it was awesome! :) DLC drop #3 coming soon.So it looks like we are still on track for the release date but that the focus has shifted from multiplayer to singleplayer. When asked about the tweet, Staley responded "It's a new difficulty mode. If you liked Survivor Mode you'll love this!"
— Bruce Straley (@bruce_straley) April 3, 2014
We don't know when exactly this DLC will drop, but I think that it is safe to assume that it will be bundled with the PS4 release of the enhanced edition.