2. Ancient Rome
Probably the easiest pick on this list, considering the scope that the Roman Empire would offer to story and gameplay, as well as the infamous schemes and political intrigue that came with the reign of the Caesars. And crucially of course, the reign of the Roman Empire was a historical period characterised with often bloody war and conquest, and that would represent a strong foundation for a future Assassin's Creed series. You could go one of two ways here, with either a Rome-affiliated Assassin seeking and killing enemy targets, either within Rome itself - assassinating key figures in order to establish a new Caesar in power, or to help quash a coup - or with the player taking the role of an occupied force, fighting for their home land - with the vigour and passion of the Celts and Picts. The failed conquest of the Picts would represent an intriguing story - given the might of the Roman army and the relatively tiny size of the Pict forces, as well as the environments provided by a Scottish setting that would suit a stealth-game.