OR..... just play Kaizoku Musou (Pirate Warriors, in English) 1, 2, and 3, and you'll have a pretty damn awesome experience of "playing" One Piece.
Frankly, Hokuto no Ken (Fist of the North Star, in English) and One Piece are pretty much the only series that lend themselves to the Musou game format, that have been adopted by the Musou franchise. Running around and mowing down armies of goons single-handedly makes perfect sense when you're playing as Monkey D. Luffy or as Kenshiro. XD