As spotted by MMO-Champion, Blizzard has filed a trademark for something called The Dark Below under the category “Computer game software downloadable from a global computer network,” and while that could mean anything, that title does have Warcraft implications.
Per the Manual of Monsters soucebook, in the Warcraft Role-playing Game — the tabletop one — the Dark Below is a hell/underworld, but it seems to have only received a passing mention in the book and thus little info is known about it. But the important thing here is that it is a term associated with Warcraft rather than Diablo.
Despite the fact that Gamescom is next week, I wouldn’t expect any announcement on this there. The last WoW expansion, Mists of Pandaria was announced at Blizzcon 2011, and we have another one of those events happening in November.