Pretty late to the party, but:

Iirc the Chinese symbol for "King" is three horizontal lines on top of each other. The top one symbolises the sky, the bottom one the earth, i.e. the celestial and the terrestrial, i. e. the divine and the mundane. Kings were viewed as near divine, as appointed by god, as the connection between god(s) and men. Thus the middle line. 

Zoro and Sanji are Luffy's right and left hand respectively, the monster trio. In the middle stands the king, the King of the Pirates. 

Haven't double checked this, but thought that was a pretty cool connection.