User blog comment:Monkey.D.Me/Is the next crewmate of Strawhat will be a Mermaid???/@comment-3266675-20101221020723

It's highly unlikely for a new member to join anytime soon. According to SBS, the only reason why the characters in the beginning were recruited so quickly was because Oda needed to created the main crew as quickly as possible. The only reason why Brooke joined a little after Franky was because Oda couldn't wait untill his planned, skeleton man to come out in One Piece.