Yeah, the only way I can see Zoro becoming a Captain is if Luffy specifically orders Zoro to become one.
One of my very earliest story ideas set in the world of One Piece - for serious, I was but a wee infant fan barely able to toddle to her phone and then be immediately insulted by the lack of Zoro/Mihawk - actually delt in this "trope". [I was also set pre-Skypiea, so at this point I had still mostly seen the filler arches and let me say.... Skyiea did very little to change my opinion on that matter.]
In basic terms, the idea involved the Strawhats forced to sail through this series of closely linked islands, each with their own little societies of very gossipy but very different - at least in the native's opinions - cultures/morals/cuisine/whatever (alternative they could have just been rural populations of very dedicated bounty hunters; we're talking backwater enough that they would have been shocked to hear that whippersnapper Shanks made captain much less Yonko).
You would think such a place would be paradise for pirates on the run, but the truth is, these Islanders may rarely get flooded with new bounty posters of Wanted Men (and Women, and Fish-people and Mers and Minks and listen these people don't discriminate against a criminal with a price on their head) but that hardly stopped them making and spreading their own their through little archipelago whenever a clear threat was so spotted.
Look, it wasn't Luffy's fault he may have accidentally on purpose destroyed this town's entire floating fishery and aqua garden - the only one they had available in their rather unreasonable mountainous winter island habitat - but on top of the wanted poster someone finds stuffed in his back pocket, it spells death.
Luckily, they do have Robin around at this time, and she knows just enough about these islands, peoples and customs to have just the solution.
Make Chopper-San captain! After all, who would expect such a brilliant, caring doctor to be a horrible, criminal pirate captain!
Arg, Robin-san you idiot! You can't make him happy just by saying he'd be such a good doctor captain!
Surprisingly, it's not Nami who takes issue; she may not trust Robin like "the boys" (because she "has boobs and funny powers" *sigh*) because she Also recognizes that Robin is, like her, a smart surviver, and that she is willing to support (well at least until it becomes dangerous and let advantageous). So Nami's vote goes to Robin's plan: have Chopper act as captain until they're safely out of these crazy waters. Of course, as the plan came from the intelligent Robin-swan and has the approval of the beautiful Nami-chan, Sanji wouldn't say a bad word against it, but you can tell in the way he eyes the horizon he's preparing himself for the inevitable trouble.
Then of course, there's the matter of convincing Luffy who,
"Hmm? You mean like playing pirates! Oh, I call captain!"
"Luffy, you can't be captain, the whole point is that Chopper has to be captain so they don't come after us like that again!"
"Hmm? Okay, well then I want to be......Oh! I know-"
And just like that he runs off to get whatever supplies he needs to "play pirate", leaving a cheerful crew (and one very happy reindeer, currently beaming as Robin fits him with his own little captain hat and Usopp takes out an old bit of sail, asking what he wants his Jolly Roger to look like.)
The only one completely unamused is, of course, Zoro is against it on principle (even though he thinks Chopper looks sweet as a baby angel in his captain's hat, I mean just look at their doc!) Oye, where's she get off, telling the crew who their captain is!? What is that women even on about? Zoro and the Merry only have one captain and they can't just go changing that on her whims! If they get to the next island and people are still pissed, just let him and Luffy fight them off! That's what THEY do on the crew!
But it's too late, Luffy likes the sound of this game, so from now until after the next island they're officially the Cotton Candy Pirates!
But of course at each island something goes wrong and in a panic they switch to another captain.
Each rotating Captain gets the whole expierence:
☠️ Naming and "recruiting" a crew, though that last one is more just assigning everyone a roles (Luffy insists it's absolutely necessary for any pirate captain, even only a temporary pretend one; though sometimes it's a necessary swap when the townspeople ask why their doctor looks so much like that doctor pirate captain they just heard of from the last island.)
☠️ Their own Captain's hat, of course, and sometimes the full Captainly ensemble to match (don't ask from where, from the same jumbled pile of clothes they pull all their other outfits, okay?)
☠️ A new flag adorned with their own Jolly Roger (courtesy of Usopp)
☠️ A banner on the side of the Merry (again, Usopp, with some help from Chopper and Robin) with their new crew name, just to make it clear they aren't the Strawhats.
☠️ Something inevitably go horribly wrong, forcing another quick disguise change for themselves and Merry before the next Island.
In case you're wondering.... (You weren't, but you're gonna suffer through it.)
Chopper takes his role very serious, which means he has lot of fun since he basically gets to play pirates with his best friends! Usopp makes him a flag, Robin finds him an awesome costume, Sanji calls him "Doctor Captain-san" - Usopp said his name should have been scarier but Chopper didn't want any of the kids to get scared of doctors even though they only had to stay on this Island for six hours for the log pose to reset - and gets him a sweet whenever he asks (except when Nami, who claims she is playing the ship's OTHER doctor, says that he will give himself a tummy ache). Luffy beats around on a bunch of pots and pans which the others hate but he says he has to because he's the musician and that all pirate crews have a musician (Chopper wonders why they don't have a musician, but he doesn't think to ask at the time). Since Usopp offers to be the new navigator Chopper asks Robin to be his first mate, and then asks Zoro please if he wouldn't mind being the crew's army fighter protector, since he's the strongest person Chopper knows. He doesn't play with them very much but he does smile just a little when Chopper asks so Chopper thinks he's a pretty good captain!
That is until he talks to some poor weak looking lions that are all beat up and come sniffing around the ship for scraps. He tries to give them some medicine, but the town folk don't like that at all. They have to leave really quick, but as Captain, Chopper decides they're taking the lions with them to the next Island, where hopefully they'll be safer!
Usopp goes next because, well, he's the one who convinces the next island that of course they aren't those cowardly, evil Cotton Candy Pirates! Haven't they heard of the great Usopp Pirates before, led by the famous captain Sniper Warrior Of The Giants Kyoyūshi Usopp! In fact, these are lions they recently saved from some totally different terrible pirates the, Errr, Strawhats, who you probably haven't heard and certainly don't need to worry about, not after Usopp took care of them! They had been abusing these poor, sick lions and forcing them to go to towns and scare people or they would be beaten and starved.
Fortunately, Usopp happens to have world famous linguist on his crew, Chop-Chop. After he ate the Tenuki no mi, he can even talk with animals, and so Usopp gives the people his solemn word as the strongest, most trustworthy captain in all the seas these lions won't hurt anyone in the town. Indeed, they've promised to work alongside you fine citizens while hunting and even guarding you from pirates, just so long as you promise to share some of what they hunt and don't beat them or chase off their young! Their doctor, uh, Mizzie Sunday, has plenty of experience as a veterinarian as well. She'll be happy, with Chop-Chop's assistance, to help get both lions and people set up and comfortable with this prosperous new alliance!
Yes, it's true, Usopp's crew is quite amazing! You should meet, um, Cat! She may look pretty and sweet but actually she's one of the most fearsome swordsmen of all times! Huh? Oh, well, she often makes him hold her swords, so she can decide which one is best depending on the battle. That's how amazing she is! And she trusts Zo-Ro-Zo because as their cook he is excellent with knife care! Oh, this is his first mate and trust advisor Yonji - eh? Haha, right now he forgot you hate telling people your first name! (Why would Sanji hate the name Yonji so much? What did Usopp say?) They simply call him, The, uh, Heart Breaker, for the wherever he goes women fall over themselves, helpless at their charm!
And finally of course, this is our bard, the man in charge of chronicling my many amazing and legendary deeds! We call him Monkey but he has many names, they say no one has yet to know them all!
So if you could just do us a favor and -
Usopp spent most of the next four days talking. Of course they ended up in trouble by the end!
I could go through each and every one one of these crews but let's just say when it gets to Zoro it goes something like this.
Zoro: So... I'm the captain, now?
Sanji: *trying not to look nervous after what he had Zoro do*
Chopper & Usopp: *jump to attention*
Robin: Yes, Captain-san, we await your orders. *Smile*
Luffy: *lies on his favorite spot, watching Zoro curiously*
Zoro: *Nods, seriously* Good.
Zoro: Oye! Luffy, get your ass own here!
Zoro: I'm making Luffy captain. *Throws Luffy the ridiculous cape Usopp had given him* And since I'm captain no one can do a damn thing about it. *Stocks off to go get in a much needed nap until Luffy inevitably causes a fight to break out*
The End