Positions/Roles of current Strawhats:
Captain, Will of D. Carrier
Combatant, First Mate
Navigator, Quartermaster, Witch
Sniper, Gardener, Inventor
Cook, Royalty, Female Bodyguard
Doctor, Pet, Animal Translator, Scientist
Archeologist, Spy
Shipwright, Engineer, Cooper
Musician, Ghost Corpse
Helmsman, Lifeguard, Firefighter
Listing out the Strawhats ship roles and subroles gives a better picture on what may be missing from the crew. There are conventional as well as unconventional roles that can be demonstrated or have story relevance.
Ship Guard
Lookout
Apprentice
Cabin Boy/Girl
Ship's Cat
Chronicler
Powder Monkey
Nightwatch
Master-at-Arms
Boatswain
Sail maker
Scout
Striker
Strategist
Rigger
World Government Agent
Marine
etc...
Some positions or roles could be attached to a current member or even multiple. So if a new crewmate were to join than new crewmate would have to demonstrate they are better than every other crewmate at said role or position.
If someone else joins, which I do believe, then this future crewmate should be as useful as Jinbe, IMO. Jinbe, as a veteran, covers major needs for the crew. The candidate that covers the most potential important postitions and roles is the best candidate, IMO.
Below are some candidates and some roles they fill. List is not final. Add or subtract what logically seems suitable.
Vivi: Royalty
Kuma: Ship Guard, Revolutionary
Bonney: Cabin Girl
Vegapunk: Scientist, WG Agent
Stussy: WG Agent, Spy
Lucci: WG Agent, Spy, Shipwright, Ship's Cat
Kizaru: Marine, Scout
Smoker: Marine, Lookout, Master-at-Arms
Yamato: Ship Guard, Apprentice
Carrot: Lookout, Apprentice
Tama: Ninja, Cabin Girl, Monster Collector