Thank you for the thread, I have been waiting to discuss this in depth and have a lot of thoughts about it. I was so excited when i first heard the news and been planning to discuss it in detail. To begin with, atleast the good news is its a TV series, a Movie would've been an instant failure... If done well, it can reach the success of GoT, or atleast exceed the current comic based TV adaptations..
Here are a few critics in my opinion of things, when compared to similar ideas and how One Piece TV adaptation can fair in comparison.
One of the things Hollywood, or even the west is accused of is Whitewashing. DBZ was terrible, so was The last airbender. Even Ghost in the Shell was a flop. All these movies were adapted from Manga / anime but the characters were all so poorly chosen that fan backlash already ruined them even before the casting.
However, One Piece can be free of these accusations. Oda has already stated which countries the strawhats would originate from, already giving an ethnic background to them. So at least the casting should be free of this accusation, atleast for certain characters.
I also think One Piece offers a better platform for a live-action adaptation than other fantasy genres. While Naruto, Bleach or DBZ characters can look like an eclectic mix in casting, a lot of the narrative and even location seems closely tied to Japan. One Piece creates this world of fiction with incredible detail which, if adapted correctly can appeal to a larger audience and specially with the adventure narrative. Everyone loves Indiana Jones, Pirates of the Caribbean, Back to the future etc, and I can see One Piece have a similar narrative mix of light hearted humor and a serious plot to go along.The linear structure of the storyline also helps align with the many arcs so beautifuly, Season1 can be east blue, season 2 grandline till alabasta, etc.
One thing would be tough is to please the anime fans. No matter how good or talented the actor is, we all have a defined expectation on how the events will unfold, how Luffy's expression would be etc. That will be hard. Even so, the audience if disappointed with the TV adaptation, they'll compare it to the anime which will draw a larger audience, so its still a big deal for Oda.
I am excited for this, hopefully HBO or Netflix pick it up..... Any ideas on the casting?