It seems that One Piece: World Seeker will not come out until 2019. Though that game was set to release in 2018, Gematsu had found news about it. In the latest issue of One Piece, #90, game is listed as coming out in 2019. Manga provides no further information about release date.
One Piece: World Seeker will be an open-world game about adventures of Luffy and Straw Hat Pirates on the vast oceans.