User blog comment:AssassinRome/Assassins in Japan?/@comment-3058930-20120805212727/@comment-1153722-20120806073741

Tokugawa Ieyasu unified Japan and brought peace to the land for the next 250 years or so. Him being an antagonist wouldn't make sense, as that would mean the Assassins would become enemies of the Tokugawa shogunate. Ieyasu could serve a similar role to George Washington instead.

Oda Nobunaga or Ishida Mitsunari would be more sensible antagonists, I think, or a random general in the Western Army, perhaps Kuroda Kanbei? I suppose Toyotomi Hideyori at the Osaka Campaign could make for a good final battle too, but I believe he was around 18 when he committed seppuku alongside his mother Lady Yodo, so he wouldn't really be the "main antagonist" throughout the game.

Oh, and you also forgot shuriken. :P