10 Places Assassin's Creed Could Go After Valhalla
2. Sengoku Japan
If there's one setting that fans have been clamoring for since beginning, it's Japan. It's been the one thing that has seemed to unify the chorus of pleas from the fandom. It's ironic, too, since the most ideal setting features a Japan that's anything but unified.
Indeed, the ideal situation would see the player dropped into the middle of the Sengoku period (A.K.A. "the Age of the Warring States"). This is the era people picture when they say, "Why hasn't Assassin's Creed gone to Japan?! This is the era of samurai, ninjas, and feudal lords. This is the era of Oda Nobunaga, Miyamoto Musashi, and Hattori Hanzo.
Basically, the Sengoku era saw Japan plunged into a period of perpetual civil war after the collapse of the Ashikaga Shogunate (and the feudal system, along with it). Warlords and samurai clans clashed relentlessly for control. Ninjas were employed to sabotage and destabilize enemies.
One must imagine that your assassin character would be a shinobi, as ninjas are arguably the most iconic assassins in history. The days of the ninja came to an end with the reunification of Japan under the Tokugawa Shogunate. It would be a cool way to explore the psyche of a group as their role in society disappears.