Like
@Beerbottle mentioned, you get to create a character, join a organisation. Then go out to sea as either a solo character and faction or part of a team. As you go further into your adventure's completing them, you gain rewards and exp. The more you exp the closer you can go to getting your character a certain DF or power up.
Eg: could be using your UN, Alibaba in the one piece world. In his own faction or whether a pre-existing faction such as the marines, revo, cipher po, bounty hunters. Or create his own faction or sub-faction under a faction. Then maybe as you get your character to level up you can then upgrade say your dagger/sword to a cursed one that can engulf to flames of amon.