Hey there, Fury. Welcome and thanks for your interest in BSR
Currently, the most recognizable open role on the ship is for a Companion, someone who, like Inara on the Serenity, would use one of the ship's shuttles as their living quarters and place of business. As for the other two roles, they can be either passengers or crew. With crew I like for each character to have a 'specialty' or niche skill that lets them fulfill role on the ship and allows them to shine in unique ways in gameplay (though there's always going to be some overlap among the various gunhands). The taken roles I have listed in the second post are only the ones I could think of off of the top of my head. There are certainly more that we can come up with together. More and more, looking at the character sheets, it feels like the crew is becoming the Captain's private mercenary company, lol.
Passengers can be pretty open as far as background and reasons for traveling are concerned.
Was that at all helpful? You're more than welcome to PM me for a lengthier discussion.