At this point in the story, who cares about position in the crew anyway. OP is not a MMORPG where everyone must fulfill a designated role! Heck, in raid against Kaido and Big Mom there isn't any healer!!
(Well, Law CAN, but even he can't insta heal like Mansherry)
Look at other pirate crews, such as Law's Heart Pirates. They have 20 members but only Law and Bepo has designated position. Jean Bart doesn't even have clear job beside being a fighter.
Or Blackbeard Pirates. Besides the original 5, their recruits from Impel Down don't have any ship position! What is, for example, Catarina Devon or Pizarro's ship position? Who is their chef? Shiryu is yet another fighter. And they're just Titanic Captains nowadays.
My point is, Carrot, Yamato, Tama… all of them can become fighters!
IMO what the crew really need now is an extra firepower. They can all provide that. After Wano, Luffy is on the level of other Emperors, they will fight Shanks, Blackbeard and the other heavy hitters of OP world! They need all the power they can get!
Not that I argue that all 3 of them will join anyway.
By the way, if Vivi rejoins, what is her position? Princess? Dancer? Duck-Rider?