Welp, **** just got real.
Eidos has registered domain names for "Deus Ex: The Fall" (God Out of the Fall? What is this, a Suda51 game?). No confirmations of any kind, but with two Deus Ex titles and one confirmed film, that can only mean one of the two is the game.
'The Fall' would fit a game set between HR and DX, as it would demonstrate the fall of the glossy world of HR into the dark, decadent cyberpunk of DX ("It's not the end of the world, but you can see it from here").
For those of you worried that Eidos Montreal may be stretched thin by working on this and Thief simultaneously, keep in mind that EM is composed of at least two (likely three) teams, and none of the HR members have been spotted in the Thief coverage trailer at Game Informer. In all likelihood, the HR members were just shuffled into the next DX game.