Think they can get a good casting in East Blue, most of the characters are kind of easy, but they will have to cast some of the most iconic characters like Roger, Shanks and Mihawk very early, and some of those characters why not show up again for years, if the series have other seasons, what i dont think will happen.
But casting is the easy part, what i think will be most difficult to make the "feeling" the OP world have, that it is anything but realistic, some things just look stupid if u put humans, but in a comic media they look just funny at most.
Pehaps they will heavily adapt and make it less fantastic, what i find better, than try to replicate things like Kuro gloves, Pearl, Moo moo, Gaimom and other things that just dont work on live action.