This is the official info we've got so far(as far as I know)…
Covered Arcs: Buggy, Baratie, Arlong, Drum Island, Return to Sabaody. (There was also an image in one of the first ads of the game that seemed like Impel Down, and in the first trailer Marineford.)
Playable Characters: Luffy, Zoro, Nami, Usopp, Sanji, Chopper
Bosses: Buggy, Don Krieg, Mihawk, Arlong, Wapol (Kuma android in gameplay video doesn't seem like a boss)
The game is a Warriors(Musou) title, so it will have tons of enemies and repetitive(but fun for some, like me) gameplay. The game seems to have a different combo system from other Warrior titles, instead of pressing square a couple of times and ending the combo with the triangle it now seems that you can mix and match both buttons to get different results. It also seems like you can charge certain attacks. The camera also seems to zoom out when doing certain attacks. There will also be many quick-time-events (QTE) better known from games like God of War, and for those of you fond to anime game titles, the Naruto Ultimate Ninja Storm series. Which make you feel like taking part of certain cinematic scenes. The game also seems to have a "partner" system which I believe Warriors Gundam had, where you can summon a character to help you in battle when in need. There also seems to be some kind of "platforming mini game" involving QTE when seeing one of the trailers, where Luffy appears jumping and grabbing poles using his devil fruit ability.
That's all I know for now from what I've read, seen, and heard (on OPP). IMO, is the best looking Musou game I've seen in my entire life, and I've played pretty much every Musou title.