@BeariousJones:
-Lookout could want to see the whole world. Maybe a writer. Or astronomer. Some interest that is sightseeing-ish. It also has been stated that Zoro and Robin suck at lookout if I recall correctly.
-Helmsman could want to steer through the most dangerous whirlpool, mountains, volcanoes, or etc. The New World I feel needs to show the difference of the regular traveling dangers. Needing a person that can drive through all these obstacles safely should be a must.
I think if only 1 or 2 more crewmates join they will be one or both of these two. Not sure if any people before the time skip fits those roles. I am leaning on new post time skip characters joining.
Lookout seems a little redundant with Usopp, especially since he can now CoO people at long range. Robin and Brook both have good ways of scouting something suspicious as well, if they need to. Someone nocturnal would be useful however, since I get the impression the whole crew goes to sleep at night.
I do like the idea of an astronomer, but I think we'd have gotten more direct hints of that being useful if the crew was going to wind up with one. Honestly, I wouldn't be surprised if Robin winds up covering that role later in the series, especially if the cosmology and history of the world are closely related.
Helmsman also seems a bit redundant with Franky. I've soured on Jinbe taking that role for a while now, simply because he's probably more useful taking a more direct role in steering the ship–namely, by getting out and pushing from the water. I can't think of any bad weather situation where that's not the most effective use of your water-bending super strong whale shark, and Franky is perfectly capable of piloting the ship himself. "The Water Guy"/Frogman is probably a good enough role for the person who is probably the strongest water-fighter in the world.
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@Ouroboros:
Ivankov should be the next crew member, so that he can turn all the Straw Hats into women and then no one will complain about "TOO MANY MEN ON CREW" ever again.
Wasn't that a TV special? If not it should be.