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Bet 50 on the fact that Ubisoft needs to cash in a pretty big Assassin's Creed and the Roman Empire is sufficiently similar as environment to the first three games while offering the chance to reuse parts of the Italy, Greece, Britain, Egypt and France environmental research. Could also be a reason not to do it, as it could look too similar to the previous settings.
@Matteo62fb I suspect that's what they have in mind for the later codename invictus, but yeah that would definitely be cool, would love to explore a version of Rome resembling the Course Of Empire painting

Despite sounding obscure this might be the most likely since the codename for their next project is hexe, German for witch, and witch trials peaked in the 1500s-1600s
Of course, there's a chance the code name and teaser were red herrings, since AC Shadows ended up having little semantic connection to codename red, and also a chance that it might be 2D (like AC Chronicles), not on Xbox (like AC Liberation), or even not a game (like the AC movie)