Last week it was discovered that Square Enix had registered the domain for something called Deus Ex: Human Defiance, but it turned out to only be related to the upcoming Deus Ex film. Fortunately, an all new Deus Ex-related trademark has been discovered (thanks, Superannuation), and it suggests Square Enix is working on a project called Deus Ex: The Fall.
The publisher has currently registered domains DeusExTheFall.com , DeusExTheFall.net and DeusExTheFall.co.uk. Could this be a new Deus Ex game Eidos Montreal is working on? Or perhaps it's a reference to the upcoming enhanced edition of Human Revolution, although you'd think that would have a more recognisable title, like say, Deus Ex: Human Revolution: Special Edition. The Australian ratings board leak regarding that suggests it will bear the familiar Human Revolution moniker, too, so a new game is looking more likely.
Source: http://www.eurogamer.net/articles/2013-03-06-deus-ex-the-fall-domain-registered-by-square-enix