Not arguing with you or anything just throwing something out there. From what I've seen の is used more as a 's than as of. So it's really be Sun's Pirates .
Uh, it depends on context dude. Japanese の is a multipurpose use.
You see it used as "'s" more simply because you see it used often after a pronoun in the context of belonging.
Obviously "Pirates' King" or "Strawhat's Luffy" doesn't work due to the context.
If anything, you should know that Japanese is not a precise language, and is very context driven.
Luffy of the Straw Hat
That's actually close to the feel of it, but obviously it sounds silly in English.
Exceptions have to be made when it doesn't work out in English, but in cases like "Pirates of the Sun" where it works fine, might as well use it.
Kinda sounds like a certain Disney franchise.