Why not Helmsman? The fact he should work togehter with the Navigator makes it more common sense he should be the Helmsman a Fishman and Nami controlling the ship. Yeah that's some sort of sign imo… for the readers :).
Other than that it is as good as confirmed that Jinbe joins so even if you don't take like it. It will happen, the story went all the way to that moment.
Anyway people saying about how they don't like talking about Nakama, tbh. That is one of the good points of One Piece, you keep guessing who joins and who doesn't. That is one of the points it makes the serie so enjoyable. Not many manga's use this method the way One Piece shows it. It is perfect and I hope more manga writers will bring this way of writing new characters to us after One Piece ;D.
Anyhows like I said with my previous post and my theory: Jinbe joins as the physical fighter, A Samurai as our new swordfighter who uses 2 style swords (maybe Kinemon or his son), Bonney and last the Cabin Boy maybe?