It just wouldn't work, unless the series was transported to the mid 20th century or later (or maybe even near-future). As previous posters have said, the only female assassins in history have generally been informants and seductresses (which would make for a game which is simultaneously X-rated and dull as hell). Inserting a female assassin who is your typical sassy, empowered, backflipping femme fatale into a historical setting would be anachronistic, not to mention completely gratuitious.