When thinking about the current crew, and how they all got there...
Some were literally called out in the story as a need before they joined:
- Nami - Navigator - When Luffy and Zoro realized they would just float around aimlessly otherwise
- Sanji - Cook - When Yosaku was malnourished... plus Luffy needs/loves to eat
- Chopper - Doctor - When Nami was sick and everyone realized she was the only one who slightly knew anything about medicine/health (see note about Yosaku from above)
- Franky - Carpenter/Shipwright - When the Merry was dilapidated and they realized their next ship would probably receive the same fate, since they are so hard on them apparently
- Brook - Musician - Luffy was only asking for/prioritizing one over literally every other position since pretty much he set sail
Others were not called out in this way, but may make sense in a traditional crew and parallel other crews, specifically the Red Haired Pirates:
- Zoro - A pirate crew needs fighters (swordsman), main muscle/first mate akin to Benn Beckman?
- Usopp - A pirate crew needs fighters (sniper), akin to Yasopp
The others remaining seem to fulfill other, unique purposes:
- Robin - Almost a parallel to Oden from Roger's crew. Luffy could not reach the One Piece if he did not have someone to help him read the Poneglyphs. Roger knew of this need when he recruited Oden, Luffy did not know until later
- Jinbe - Maybe trickier. It helps that he can steer the ship well. Maybe more of a cultural implication in helping to tear down the divide between Fish Men and Humans. Certainly helps as a mentor/veteran perspective as well and credibility/muscle as a former Warlord
The "final" member feels like they would have to fit in to the "unique purposes" bucket as nothing has been explicitly called out from a "needed position" standpoint in quite some time. I guess more muscle for the sake of more muscle could work too, but feels like there will something more to it.