If she's mentioned what that era is somewhere in the past eight years we might have an answer.
Source: http://www.adweek.com/news/technology/meet-woman-behind-assassin-s-creed-155897AdWeek said:Theres a compelling story propelling Assassins Creed, as well, one that came from Raymonds fascination with conspiracy theories developed from reading books about the Illuminati in her early teens. Assassins Creed borrows heavily from that genre, featuring a secret society of warriors who travel back in time (not by time travel because thats cheesy, she offers) using genetic memory (this idea that the memories of peoples ancestors are encoded in their genes).
Assassins Creed has explored the Crusades, the Renaissance, American Revolution and most recently pirates. The fifth installment will feature her favorite historical era, Raymond says, but she cant even give a hint of what that is because it would ruin the surprise. She did dispel the rumor that it is set at the end of the samurai age in Japan. That said, she thinks that would be a cool idea.
When were building a franchise, were thinking of a whole universe and how were developing a meta-story that could live on for many years in games and TV, explains Raymond. If the story has nowhere to go, then you end up with the video game equivalent of The Matrix 2 or something where its never going to be quite as genius.