http://www.ign.com/articles/2013/02/07/next-assassins-creed-has-new-hero-time-period-developer
The next Assassin's Creed title will take place in a new time period for the franchise, and stars a new protagonist. Ubisoft CEO Yves Guillemot said the "all-new Assassin's Creed hero, within an all new team and time period" will be "another major leap forward for the franchise" when it releases some time before April 2014.
Apart from the fact it stars a new character (praise be to Talos), and a release date "before April 2014", nothing's known about it. Not even what systems.
EDIT: It might be pirate related: http://www.ign.com/wikis/assassins-creed-4