I think the only really relevant positions pertinent to the debate are the ones that are analogues of what we've seen on Blackbeard's crew. Since Helmsman is one of them and the Straw Hats didn't, and Jimbei is trained to be one, it's kind of a no-brainer unless a heretofore unknown one comes along which…why? Just why? Not that anyone asked that, but it feels like it's just beneath the surface waiting to be implied. Jimbei's not just a normal Helmsman anyway, he can turn a ship into a windsurfer, and even from a position not behind the wheel, he can push from behind or tow from the front if the ship gets into a rough patch that it can't pull itself out of with just the wind in the sails or if they're out of cola for a burst. His being a Fishman multiplies his capability as a Helmsman because of the natural talents that come along with it. Dealing with storms and other hazards is something the crew hasn't had to deal with very much as of yet and it's likely to change once they get close to Raftel, so having Jimbei will be much more important.
I'll concede that, while the utility of a lookout will also become more important, it feels odd to have saved that for this late stage of the game, and even more odd for it to be speculated to go to someone who has no long-range capabilities to deal with anything they might scope at a distance; that's Usopp's specialty and given his nature as a sniper it just seems redundant to have a second set of eyes. If there is to be one more member of the crew, I'd kind of like to know of a position more substantial that would make sense for them to have, and since none is yet presented I'm thinking it'll be some kind of more story-relevant wild card which feels like Yamato, given her family connections, might be the best option for. That's absent any other better data, and considering the backseat Carrot's taken for a lot more of the arcs she's been hanging about, I'm not sure what the hell Oda is trying to do with her, but it's not painting a flattering picture nor justifying high confidence in her chances.