See, the thing is, the Pirate Motif sounds really, really cool. The early chapters of Shanks and Luffy make for a great set-up from a pirate perspective, not unlike any of the PotC films. Bandits, revenge, destiny, sea monsters all could work.
The super-natural or super-human stuff is where it'll feel tacky. That stuff ALWAYS feels tacky, even with a budget. Not even super hero movies can pull it off that well. Cap and Iron Man get grounded hella easily with combat costumes, machinery, tech, gadgets, and gizmos. Once you go beyond into actual deformation, and super power fuckery, you might as well get Willy Wonka on the phone.
The human elements, a.k.a. the core of One Piece, can be done, but I'm worried about pulling off the translation. Somehow, whether subbed, dubbed, or translated, you get the vibe of the world and DEFINITELY how Luffy, Zoro, etc. all these characters act. - their dreams and hopes. If you give it to (presumably) Western actors reading lines from an altered script, are they going to bring the voices to life? Cause I'll tell ya, the VA cast has already nailed that part for me.