I've just heard this info from a friend who's working as an insider developer at Ubisoft (mind the term insider, this means he knows info about the next 10 installments)!
The story will take place in Germany, mainly in Bavaria, in the city of München. The time period will be 1810-1850, this is the time when the Assassins were fighting the Templars for control over the first Oktoberfest (or Beerfest if you want another common name for it). You'll control Klaus Jürgen aus Hamburg, a fat german Assassin who'll have to find the six secret Beer recipes hidden by Ezio Auditore da Firenze. Gameplay will change drastically as a fat dude can't really climb on buildings and stuff like that, but don't worry, you'll have a mobile ladder in your pocket which you can use to climb on buildings. Also Klaus can't move too fast because of his enourmous beerbelly and can't hide in crowds, so you'll have to use disguises (for example you can dress up as Bavarian waitresses thanks to a special Assassin corset which transforms your belly into bosoms).
You'll have a lot of special weapons, for example instead of poison now you'll have the ability to shoot darts full of concantrated alcohol making the enemy soldiers drunk, sick or simply killing them (depends on how many times you shoot them). Of course you'll also have to worry about the economy of Munich, but instead of building the shops of the city you'll have to renovate the pubs and beer factorys to control the distribution of the precious ale. Also you'll have to capture so called "beer towers" controlled by the Templars which are basically water towers but with beer in them!
All of this sounds fantastic, I can't wait to play!