Personally I think there are only a handful of places left to visit that could allow for a Assassins Creed style game. As each game moves forward in time, the possible era's to visit are decreasing. Personally I think there are several that could be the next setting.
~American Civil War: Muskets and revolvers were standard, yet still not advanced enough to impede gameplay elements. Calvary and sabres/bayonets still allow for melee combat.
~Napoleonic Era: Muskets, Calvary, and Sabres. A lil lower tech then the Civil War, but still a possibility (Considering that all the Assassins Creed games have been European based so far.
~Japan: Limited firearms such as muskets, Calvary, and lots of arrow/sword fighting. (The time would be roughly when America first contacted Japan during the so called 'Final Days Of The Samurai' when new technology and outside culture began to get rid of the need of Samurai or ninja. (Era could also be earlier when the Portugese and Dutch were trading cannons and muskets to Japan)
~World War: Highly unlikely, but a side story (Web Novel/Canon) mentioned that there was a Allied doctor who during the Xmas ceasefire with the Germans went to go patch up a German officer. The German turned out to be a Templar, and the Allied Doctor (Assassin) killed him.