What a nice feeling to know about the next One Piece movie! And yes, it's the 13th movie, and it's labeled as a One Piece Film! It's important, and here is why!
Just announced on the One Piece official website, the next One Piece movie is announced to hit the big screens in the next Summer (2016). The official website compared this movie to Strong World and Film Z, the 2 biggest and most important One Piece cinematographic releases directly related to the main story, where Oda Eichiro is directly engaged, they are also the only movies labeled as: One Piece Film(s) and since the new one has the same label, we can presume that THIS IS ANOTHER BIG IMPORTANT ONE PIECE MOVIE!
There are no much details for now, just the release date, not even the full name. Also for the comedians, they only announced the 9 actual Mugiwara voices actors. More details should come very soon! Stay tuned guys!