Seriously. This is an hell of a good idea. I really like it. And now i know it, I wish this is a thing in WoW.
Maybe you just start as a regular Alliance or Horde character and at level 10 you can visit some guy in Booty Bay (Bloodsail Buccaneer in disguise) where you can start a questline to prove your worthy to be able to join the newly created Faction 'Bloodsail Confederation', of which the Bloodsail Buccaneers would be part of. And so are other pirate crews, like that Keg Leg's Crew from Plunderstorm etc..
Ofcourse you wouldn't be able to visit Alliance/Horde towns/cities. So you get a new capital city for all your city needs inspired by Nassau (Black Sails)/Tortuga (Pirates of the Caribbean). Perhaps Bloodsail Hold on Plunder Isle could be introduced for this??