teaser page on the official Diablo 3 site looks to point towards an expansion called 'Reaper of Souls.' The page shows an image of a looming, shadowy entrance, along with a quote from the Book of Tyrael, which reads "Death, at last, shall spread its wings over all." The page itself is titled Reaper of Souls, and text at the bottom of it indicates Blizzard trademarked that term, something TMView confirms happened just yesterday. Also, the image has the filename "d3x_teaser_en_us.jpg," the 'x' there suggestive on its own.
We know a Diablo 3 expansion has been planned for some time, but this teaser page adds to suspicions that more info will be forthcoming soon, most likely at Gamescom later this month. In a recent earnings call, Blizzard President Michael Morhaime said the company would share "Diablo-related news" at the event, while invites for its press conference there teased a "special announcement." Blizzard also trademarked 'The Dark Below' recently; that now seems likelier to be related to World of Warcraft or something else entirely.
It's worth noting the teaser page quotes Tyrael's companion book for the game, which is due to be released on October 1 and promises "never-before-known secrets" about the Diablo universe. The teaser also comes just ahead of the action-RPG's arrival on PS3 and Xbox 360, set for September 3, while the PS4 receives the game sometime next year.