I mean, the Devil Fruit numbers thing has been fulfilled by all Straw Hats except the numbers 2 and 9. Along with being addressed by Oda, I'd consider this one of the few Illuminati theories that's legit.
Maybe. I doubt it actually pans out, but its so utterly ridiculous and yet so perfect that I can't help but wonder if it was intentional. If its not its one of the most oddly specific coincidences in Manga history.
Though I think the missing piece is either Kuma's DF, Kinemon's DF? The former is probably too strong to gel with the crew dynamic (though the Vegapunk connection is a wild card, especially since it opens the possibility that someone else winds up with the Nikyu), and Kinemon is unlikely if not impossible. There's a few more that would also theoretically fit, but I'm forgetting them right now (The Meat-Meat Fruit I think)?
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I always figured Chopper's number would come up due to him having a chance to stay in some sort of animal kindgom where he fit right in, and lo, Zou fits that bill perfectly. Totally possibly he gets his when they all depart ZOu for good… but that could be a few years after the Wano arc.
As for Robin, she's pretty tied into the revolutionaries. So similar thing, especially now that they've lost their base and its being made even more clear that Robin being able to read ponyglyps is important for even finding Raftel, so she's going to be more heavily desired soon.
Its also possible Oda just goofed and simply didn't give Chopper his number at the time and missed Robin when he was assigning Usopp and Franky.
And really, if he hadn't numbered Usopp then and there we'd have let it all just slide as "sometimes they just don't get the chapter title" but Usopp's led us to assume Oda had grand master plans for the other missing ones. (It also made us question if Robin or Franky was 7 until Brook got his.)
I think there's a pair of simpler explanations, which is that giving Chopper a chapter called "The Fifth" on Drum island would have spoiled that Vivi wasn't joining, and Robin's "I'm joining the crew for Realz" moment was in the middle of an arc instead of at the end of it, and giving her a "The Sixth" chapter title there would have felt awkward when she was still cuffed and in custody and most of CP9 was still standing.
I think the reason Usopp got his chapter there was more a way to give official closure to his entire Water 7/Ennies Lobby character arc, rather than a sign that Oda was planning on giving every crew member a chapter title.