The Walking Dead. That's according to an update to the game's entry in the Steam database, titled 'Walking Dead 400 Days DLC.' The name correlates with teaser clips Telltale published recently.
The four clips are titled Day 2, Day 220, Day 184, and Day 236. Each one features a person's photo pinned on a 'Missing' board, perhaps pertaining to characters introduced in the DLC.
Back in February, series writer Gary Whitta said "there may very well be more Walking Dead from Telltale before Season 2." Also, a Polygon report indicates at one point Telltale's exhibitor page on the E3 website listed 'Season 1 DLC' for The Walking Dead. While it's never safe to assume anything when it comes to the zombie apocalypse, chances are there are some more tough choices on the horizon.