IGN - Well, Ubisoft has spilled some of the beans about Assassin's Creed Revelations already. It's set primarily in Instanbul and continues the story of Ezio, now in 50s. Ezio is in Masyaf seeking out relics that belonged to Assassin's Creed protagonist Altair, so that he can relive his ancestor's memories and find the location of a powerful artifact.
Not sure if I'm a fan of the zipline concept and I would also like to know if you will be able to control assassins like in brotherhood