Just kicking the can around, but what about a Samurai who isn't a swordsman, but uses a polearm like a Yari or Naginata, or something like the Masakari, Count?
My point still stands. Fan fact though. One Piece Data Book Green shows that Nami was originally envision having a staff-axe instead of just a staff.
It is up to you to take this as indication of either there not being another staff-esque weapon user on crew because Nami seems to be centric to that idea or Oda will make an exception for battle axes.
Before it was pretty much confirmed that Jimbei would be asked to join I had an idea for a Fishman character with a fishing pole (staff/hook on a string combo, so it's a lashing weapon, one no one on the crew makes use of regularly; think Sha Gojyo's weapon from Saiyuki) so maybe some kind of nunchaku or throwing weapon(s)? Especially if it had the ability to return after being thrown…
So what you mean is a fishing rod combined with a chain and sickle? That… sounds really freaking awesome. And I know that can actually work well because the first few arcs of Hunter x Hunter focus on the protagonist, Gon, using a fishing rod as his main weapon. He would use it for grappling with the line, smacking things, stealthily catching objects from long distance, and even flipping over stone floorboards during a tournament fight. Add a big sickle sized hook as the lure and it would be quite the versatile weapon.
but honestly, I'm under the impression they're at full capacity so I'm really not interested in anyone else joining now that Jimbei is their tremendously capable Helmsman. No other position need be filled, and I'm not sure what kind of fighting style out there isn't in some way covered by the crew without using a DF which they've already got way too many DF users as it is (but Jimbei doubles as a lifeguard and is big enough to pull all four of them up at the same time if they fall in the water), but if it's to happen, they need another girl. If they stop with Jimbei, that's fine, but if they're going to add yet one more, they gotta have more ladies present to represent.
I get why you think that the crew can be complete as is. I wouldn't mind that either. However, there are two things you should take into note.
The first is that the ship role of helmsman is a fairly expendable position. It has never been essential to the crew prior to Totland in the same way that a navigator, cook, doctor, and shipwright were. And unlike every other ship role, the job of helming the ship has switched throughout the series between several crew members like Nami, Sanji, Chopper, and Franky. Jimbei is indeed the best helmsman in the crew, and maybe even the entire One Piece world for all we know. His helming skills will probably be irreplaceable as the story becomes more hectic from here on out. But Jimbei's helmsmanship role is merely a bonus. The same as Brook being a musician, but that was inevitable because Luffy kept foreshadowing that by wanting one.
Keeping in mind how an archaeologist is unconventional for a pirate ship and how One Piece Data Book Green stated that Oda originally considered a botanist for the crew (which most likely got shifted to Usopp's Pop Greens), Oda can sort of rope in whatever kind of role he wants for the crew even if it doesn't seem to be needed or is traditionally pirate-esque.
The second thing I want to bring up is that there are a bunch of clues throughout the series that point to there being eleven Straw Hats in total by the end of the series. At the end of Chapter One, Luffy said that he specifically wanted ten or so men for his pirate crew.
We probably already know by now that the Straw Hats are going to have 1v1 fights with the Blackbeard Pirates. Hence why they are dark parallels of each other. When we are introduced to Burgess in Dressrosa, we learn that he is one of Blackbeard's Ten Titanic Captains. Lafitte confirms to Burgess later on that Teach promoted himself to being the Commodore of his crew, leaving his crewmates to become captains of his ten ships similar to Whitebeard's division commanders with more of a focus on quality over quantity. However, there were only ten Blackbeard Pirates in total right before the timeskip. So Blackbeard appears to have recruited one more person over the timeskip. And I doubt it's Kuzan, he seems to only be an ally and will definitely betray the Blackbeard Pirates eventually.
Also, as irrelevant as this might seem, Oda loves soccer. And I mean he REALLY loves soccer. The one and only time he lent his voice for the anime was for a One Piece soccer special as an original character called Odacci that is the soccer game host. Soccer has eleven players on each team, which probably inspired him to want eleven Straw Hats to fight eleven Blackbeard Pirates at the end of the series.
Given how the series seems to have ten years left and how the only islands necessary to travel to seem to be Wano Country, Elbaf, and Raftel… Wano Country seems to be the ideal time to have the final crewmate join before it becomes too late for them to feel like they belong.
I didn't make any assumptions about Wano to the tune of it being more than one island, so if that was implicit in everyone else's promotion of it being the location of the final showdown with Kaido, I just wasn't privy to that information. In that sense, I'd still like to see other areas in Kaido's dominion where it might not seem like another Dressrosa, where everyone's either miserable under occupation or too Stockholm Syndrome-d to even understand what's going on. I'd assume there's at least one place under his umbrella that's nothing but thugs and punks and no 'civilians'. With all the underworld connections, such a place would be inevitable. Making a name for oneself in such a place would be a sure way to get his attention and boost their rank in his army.
Exploring one of Kaido's official territories and his HQ would be interesting in separate arcs. I won't deny that. But I would be pressed to say that Oda is not interested in that and maybe feels that Kaido's background presences in Punk Hazard, Dressrosa, and Zou, as well as his favorite island being shown in Caribou's cover story, is enough.
The place you're describing sounds like Jaya. I would love to see another lawless pirate underbelly island like that.
And I meant more like Banaro Island where Blackbeard beat Ace, and he absorbed and crushed every single building before spewing them back out. It's just fortunate it was abandoned so there were no casualties, but still. 100% of everything there gets flattened. It's a bit different when people are still there, so I doubt Oda'll do that again, although I imagine Wano's architecture being a bit more spread out so widespread destruction probably won't be a practical thing anyway. You just see a lot of criticism in other media lately about the amount of property damage and casualties and how negatively the careless heroes causing all that devastation in the effort of defending the people, it wouldn't do to have Luffy start getting the kind of reputation Kidd has because that kind of thing starts following him around. It's just fortunate that the biggest such thing lately has happened to Big Mom's tower so no one whose opinion matters will care about HER losing something.
I doubt that the typical shonen manga would care about property damage in favor of engrossing action. The only Shonen Jump action-adventure manga I can see caring about that is My Hero Academia since that focuses a lot on superpower usage being strictly regulated by the government. These stories are marketed towards young boys who mainly care about adventure and action. Oda was hugely inspired as a kid by Dragon Ball, which is famously known for its violence. It's good to see heroes trying to minimize property damage, but even if characters in One Piece cared about that, I doubt somebody as chaotically good/selfish as Luffy would care so long as he doesn't see any civilians that could get hurt. Also, Luffy has already caused huge property damage from his final blows on Arlong, Crocodile, and Doflamingo. If Luffy causes big damage anywhere, Oda will make sure to conveniently have the place evacuated of innocent civilians.
Well, there could be repercussions to this arc if somebody decides to invade Whole Cake Island and take down Big Mom after her forces are weakened from this arc's events. Whole Cake Chateau is toppled, a bunch of towns are going to be in a mess from Big Mom's rampage, one Sweet General is down and we have no idea what Cracker's fate will be since we don't know what happened Snack after he was defeated by Urouge, and Praline just took away all of the Sea Slugs that are supposed to inform the Big Mom Pirates of oversea intruders. I don't see Big Mom getting another arc to herself after this arc. At best, she gets a minor role in Elbaf but is not the main arc villain.