In One Piece, most characters eventually become relevant to the plot long after they were introduced. However, two important antagonists, Smoker and Tashigi, have been absent from the series for far too long, and mangaka Eiichiro Oda needs to bring them back in the Final Saga soon in order to satisfactorily resolve their character arcs. If he doesn't, then he will have made much of the series retroactively worse.
