Jinbe, Whitebeard, Ace, Hancock have been confirmed playable beyond the main cast.
Magellan, Sentomaru, the 3 Admirals are all confirmed bosses and apparently Enel is in there somewhere. Looks like Garp will be in too. Seems like an Impel Down stage has been confirmed, so right now every single major arc in OP from beginning until the end of Marineford seems to be in it along with their antagonists, although Thriller Bark hasn't been shown yet.
Seems pretty meaty. Could be a freaking awesome game for us One Piece fans.