I think it's highly unlikely AC could ever jump that far back in the historical time line, although their narrative is so shit these days I wouldn't be surprised.
I enjoyed Black Flag, mostly thanks to the brilliant Swansea lead character, but Unity is the same old same old and i would much rather replay AC2 than pay £40 for this rehash of tired mechanics and shoddy story.
Next setting will likely be colonial empire, i think India or South America, as that's the only place that could justify melee focused game play further on in the narative.