Astute One Piece fans have observed that the official Japanese site for the TV anime now sports a timer counting down to August 03, 2015, and a series of 8 silhouettes. Included among the images is the Japanese phrase “再集結せよ！” (“Sai shūketsu seyo!”) which suggests a re-gathering of troops or rallying action.
There are currently 9 members in the Straw Hat Pirates (Luffy, Sanji, Zoro, Nami, Chopper, Brook, Franky, Nico Robin, and Usopp) so I suspect this announcement isn’t specifically referring to the main cast. Could we be seeing the return of a group of antagonists that the Straw Hats have bested once before, perhaps?
You can view the official Japanese website here.