Even if i like the ideia of various ethinic origins for the characters, as the One Piece world seems to be very diverse, will be very hard to find good actors and make this work.
As a Brazilian i dont see any good young actor here that could be able to have such a big role as Luffy. Pehaps if the original language was portuguese could work better, but the vast majority of the good actors here are not only to old to make Luffy, but have an accent that would be very cringe to see in a One Piece series.
I agree that Luffy is very Brazilian/LatinAmerican in the way the get close to people and have very open personality, but i think in this case would be better to have a UStatesian with Latin American ancestry making him. The same for the others characters.
Another point, i dont think that a series with a Latino protagonist, and an Asian, a Black and a French(ahahah) as the main characters would catch the interest of the standart US people.