One Piece editor Suguru Sugita, who has been working with author Eiichiro Oda on his manga since 2015, was recently featured on School of Jump, an online Shueisha video program. In an interview with Mr. Sugita, the editor stated that the Whole Cake Island arc is at about its half-way point, and that the Wano story arc will begin in 2017. The manga editor also claimed that the Whole Cake Island story would have a much faster pace.
In July 2016, Eiichiro Oda claimed that the One Piece story was at the 65% mark. In July 2015, Mr. Sugita claimed it was 70% through its run.
In all, it is important to note that Eiichiro Oda and his staff have claimed major milestones, including that the story was halfway through numerous times throughout the run of One Piece, so these percentages should be taken with enormous skepticism. At first, Eiichiro Oda planned the series to be only five years. Later, Mr. Oda has claimed One Piece was halfway done before the Skypiea Arc, and did so again prior to the Water 7 arc back in 2006, and yet again in 2007 amidst the Enies Lobby arc.
More One Piece:
Jump Festa Announcements – Heading to the Reverie and Wano in 2017: