The full crew vs. not full crew argument is nonsense by the way. The cover of volume 43 doesn't include the full crew + Franky. There's a reason Robin wasn't on the cover.
It's also not just being on the cover, but also placement.
There is no logically consistent argument that says 'Carrot being on 88 ABSOLUTELY, 100 PERCENT CERTAIN, means nothing' and also 'Yamato being on the 100 ABSOLUTELY 100 PERCENT CERTAIN means everything'. You can't have your cake and eat it too.
I don't think either is confirmation. Neither is 1:1 comparable with the volume 43 or volume 64 covers which featured only Straw Hats and a future Straw Hat.
The fact is, no case has been exactly the same. It's a very rough pattern
Volume 22 - Luffy, Zoro, Nami, Usopp, Sanji, Chopper, Vivi, Karoo, & Eyelash
Full crew and Vivi / Karoo. Vivi is considered a Straw Hat, but hasn't sailed with the crew since Alabasta (though I believe she's going to return soon). Eyelash is present though, so not exclusively Straw Hats.
Volume 43 - Luffy, Zoro, Nami, Usopp, Sanji, Chopper, Franky
Most of the crew and Franky; Robin isn't present for narrative reasons. So NOT the full crew.
Volume 64 - Luffy, Zoro, Nami, Usopp, Sanji, Chopper, Robin, Franky, Brook, Jimbei
Full crew and Jimbei; Only time Oda has ever had the full crew + future crew member and no other characters, all given equal positioning on the cover.
Volume 78 - Luffy, Zoro, Usopp, Robin, Franky, Law given equal billing; Tontatta and Grand Fleet members in the foreground
Full crew present at the time. Those missing on the 77 cover are missing for the same reason Robin was missing from the volume 43 cover. In this case, the Straw Hats are separated from the non-Straw Hats aside from Law. Law being equal billing here is interesting and some have used to argue for him joining the crew. This case is sort of unique because Law has basically been elevated to main cast status without actually joining. I'd say he's equally part of the main cast as was Vivi in the Baroque Works saga. Vivi is a Straw Hat. Since the others on the cover are technically under the banner of the Grand Fleet and Law is in a formal alliance with the Straw Hats, it could make sense he has equal standing.
Volume 88 - Luffy, Nami, Sanji, Chopper, Brook, Jimbei, Carrot given equal billing; Pudding, Chiffon, and Capone in the background
Full crew present at the time. Those missing on the 88 cover are missing for the same reason Robin was missing from the volume 43 cover. There is a clear delineation between the Straw Hats placement on the cover and the non-Straw Hats.
Volumes 99 to 101 - Zoro, Nami, Usopp, Sanji, Chopper, Robin, Franky, Brook, and Jimbei lined up in the foreground; Big Mom, Ace, Luffy, Yamato, and Kaido in the background.
Yamato's position on the cover is equivalent to Ace, a non-Straw Hat. Unlike the other instances here (22, 43, 64, 78, and 88), Yamato is NOT positioned alongside the crew. I'm not saying this is a point against Yamato, but it's not a smoking gun to indicate potential crewmate potential. Inclusion here goes hand in hand with Ace, not hand in hand with the crew.
Number of volume covers on which Yamato appears is sort of irrelevant. No one is arguing that Carrot appearing on the covers of 81, 82, 85, and 88 is evidence for her joining. Law also appears on 51, 67, 68, 69, 70, 73, 75, 76, 78, and 97, but no one uses that as an argument for Law joining the crew. Law isn't even in the poll. What I mean to say is, simply being included on multiple volume covers when there's a narrative reason for that character to be included is not in itself sufficient evidence for joining the crew. Yamato appearing on future volume covers in this arc because of a flashback involving Yamato doesn't actually mean anything (positive or negative).
Law has been on more volume covers than any non-Straw Hat, and yet Law isn't considered a serious candidate for joining Luffy's crew.
There are also instances where we have the full crew and a non-Straw Hat
Volume 52 - Luffy, Zoro, Nami, Usopp, Sanji, Chopper, Robin, Franky, Brook, Rayleigh
Volume 91 - Luffy, Zoro, Nami, Usopp, Sanji, Chopper, Robin, Franky, Brook, Tama (sorry Solid; I don't think she'll join, but this is more compelling than the vol 100 cover imo)
You need to be consistent with your arguments. 'Yamato is on two covers and could potentially be on more' isn't a great argument considering there are characters like Law who have been on lots of covers and people would think that's a silly argument in favor of Law.
Simply being on a cover with the full crew is also something we have seen from non-Straw Hats like Rayleigh. It's about context. So maybe hero / villain covers are more significant. Yet we have instances of characters appearing on these covers as early as the very first pair (22) who are not crew members (Eyelash; not talking about Vivi since she's technically a Straw Hat). There are also cases (volume 43) where the lineup doesn't include the full crew. So not including the full crew does not invalidate potential meaning. So this argument doesn't work against Carrot (or Law, if you're into Law joining the crew) based on the 78 and 88 covers. Maybe we can look at positioning then? Well, that's complicated by Law's positioning on the 78 cover - equal to the Straw Hats.
What we don't have is an instance of a future Straw Hat on one of these covers present, but positioned alongside non-Straw Hats like Ace as a way of foreshadowing them joining the crew. Doesn't mean it can't happen. Not saying that Yamato being here is entirely insignificant. But it is not definitive proof. It's not a smoking gun. And no hoop jumping is required to reach this conclusion if you pay attention to what Oda has actually done rather than create an arbitrary set of rules to reach the conclusion you want to reach.
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@Monquito:
More than Carrot obviously.
O-tama is no doubt Yamato's closer competitor.
2 cover appearances, some other marketing appearances along with the crew as well, got to wear Luffy's hat, actually impacts the main story.
I'm feeling confident she'll inherit Luffy's hat to pay off for all of that, since she's definitely not joining.
At least you're willing to admit you interpret things the way you want to interpret them.