Okay, this is an interesting one. A couple of days ago Eidos Montreal lobbed up a notice they were now recruiting staff to work on their new game. To quote the statement from their site: "Eidos-Montréal is proud to announce the recruitment for our 2nd “AAA” project. … A hint! The title begins with the letter “ T”." Which doesn't mean much until you notice - as a forumite of theirs did - the previously existing line on their site here: "The first two games we develop will revive successful franchises." With the Montreal Studio already at work on Deus Ex 3, it's an easy leap to the further adventures of Mr Stealy rather than the further adventures of the-also-Eidos-Owned-Terracide. Or, less facetiously, they could be Tomb Raider... but that franchise doesn't need reviving. Let's be honest: It's almost certainly Thief. They wouldn't have said the teasing "T" otherwise.
[Thanks to Comrade Larington for the heads up]