More about adapting One Piece into this setting: fishmen. Fishmen are probably the most noticeable humanoid species in the One Piece world. Of course, this world can still potentially have Fishmen, totally, but the truth is that when you shift the focus away from an ocean-filled world the Fishmen lose their turf and lot of what makes them so major. Only replacement species I can think of are Lizardmen, who are better able to withstand unforgiving climates and lack of water than humans and also have a whole variety of special abilities to go with the different fishman species having different natural powers. Then there's haki, which doesn't need adapting but should probably be renamed to a more western-sounding term like Aura just so we don't have a clearly asian term thrown around.

As for why people haven't joined, the title isn't really that eyecatching beyond the One Piece part I guess? Even if the idea could be fun, "the alternative western story" part just sounds wierd and people often just want to play in the original world anyway. And then, I just have to say, there's your writing. You want people to always post a paragraph (which should be 5 lines, not 5 sentences btw) with proper grammar and spelling and yet your own posts, including your bio, don't really follow those requirements. First impressions like those are very important.