Interesting concept! My vague idea (subject to change if I think of something more interesting) is a cleric-type character who worships a god (not sure of the specific type of deity yet, how much worldbuilding have you done so far in this regard?) that is considered heretical, so he joins the pirate crew to live and spread his faith versus being killed. Would that work? I'd imagine a healer would be useful in many places.

Another question; regarding pirate characters from other media, do you mean like someone's OC from One Piece, or are you okay with people RPing as Luffy? (Not something I want to do, btw.)

Amyway, this sounds like a fun RP idea and I will subscribe to this thread!