It appears that The Walking Dead content mystery has been solved after a DLC listing for a new “400 Days” add-on was spotted in the Steam database.
The listing (via AGB) simply states that the content is called “400 Days” and offers no other details. Over the last week Telltale has been offering up various teasers for new content all with messages involving a certain day–the highest being Day 220–matching up to the DLC name.
My guess is that this new content will be a standalone story to tide players over until Season 2 begins.