Stephen D'Astous, General Manager at Eidos Montreal, has suggested via his twitter account that the company is working on a third, and as of yet unannounced title. In a post on his account over two weeks ago he says:
"Eidos-Montreal is looking for a new PR agency able to support a nation-wide campaign. Who's wants to be associated w/ Deus Ex, Thief and ?"
The "?" suggests that the company are working on a third game, despite also working on two of the best IPs that Eidos own. Eidos Montreal boasts well over 250 staff, so it is possible that they have a third development team at work on a new titles, but what could it be?