@Deicide Unless the story makes it clear I don't think role/profession is a good avenue for discussing future crew members. The Strawhats never needed a helmsman until WCI arc when Jinbe was shown being the perfect helmsman. Many assumed Jinbe would be the helmsman he was only really shown steering a ship briefly around Marineford arc and I think some of the evidence people cited was anime-only. Usopp didn't show off his marksmanship until after joining the crew.
Personally I think fighting styles would be a good way to eliminate potential characters. Zoro and Brook both use swords but they fight VERY differently. It wouldn't make sense to add someone like Kin'emon since he fights very similarly to Zoro. Despite me thinking Yamato wouldn't join I thought the use of a bat/club was unique. Carrot to me feels different enough from Jinbe and Sanji. Carrot being a speedy lightweight, Jinbe being a strong heavyweight, and Sanji being somewhere in the middle.