But Nobunaga is the unificator of Japan. So he must be the antagonist, based on the trailer. If the Jesuits (the antagonists in the previous games) appear in the game, the plot could be as follows:

Jesuits (the bad guys running the hidden organization in the saga) are interested in spreading their dominion to Japan.

They ally with Nobunaga who allows them to proselitize in exchange for guns and other Western goods (which in fact happened).

Yasuke is part of one of these missions and is placed by the Jesuits in Nobunaga's inner circle, as a handler/guiding hand. He is the instigator of things like the massacre of Buddhist temples and sects who politically oppose Nobunaga.

The female protagonist is a victim of one such campaign.

Conflict ensues, and she finally kills Nobunaga, but Yasuke escapes. Eventually Totoyomi and Tokugawa go on unifying Japan, albeit expelling the Jesuits.