Kin’emon’s a better candidate in every conceivable fashion. All it would take for him to join is him deciding he needs to sail with Luffy to repay him, and to find out what Oden discovered on his quest and how it relates to Wano’s past. Which wouldn’t exactly take long to set-up, Carrot’s apparent chances are dependant on a four page conversation with Pedro where he said the Strawhats are important to the Kozuki and the Mink. If Momo was older or Hiyori could fight they’d be guarantees, but as they can’t, we’re left with Kin (the first person the Strawhats met in the New World, who Luffy technically invited within seconds of seeing him) as the best shot out of our current batch,
For it to be Carrot I’d have to believe that after knocking it out the park with crewmates ten times, Oda has had a totally uncharacteristic failure. Take the last chapter for example. All the Carrot for crew mate fans get excited because she hugs Jinbei with the rest of the crew... but then she utterly disappears despite being on the same ship. Why would Oda do that if she’s going to be a Strawhat and just as important as the rest? Would that not, in fact, be the perfect time to have Carrot join in with the rest of the crew and actually seem like a part of it? It’s not like on EL Oda was stepping back from involving Franky with the CP9 fight and the rest of the crew cause it was Robin’s chance to shine and he wasn’t a Strawhat yet.
That’s been the story of Carrot. Too often overlooked, too often in the background. A three panel flashback for her apparent character defining relationship with Pedro, for gods sake. That is not a character being set up as part of the main cast.
She’s also a very one note character. She’s energetic and fun. There’s not much else to her. There’s nothing in the way of conflict with the character, she’s shown none of the darker, more negative aspects that made the other Strawhats (never mind the Strawhats, any character really) compelling characters. The other Strawhats have all shown various degrees of self-loathing, being outsiders, guilt over their past etc. Carrot’s just... happy. Then Pedro died and she was sad. And now she’s happy again. Not exactly up there with a pathological need to lie due to the early death of a mother, fifty years in isolation due to a promise or a self-destructive, borderline suicidal devotion to a promise made to a childhood friend, is it? If Carrot was to be a Strawhat Oda would be devoting time to stuff like this, how the attack on Zou effected her, her relationship with Pedro. Like he’s done for Momo and Kin’emon’s relationship with Oden. He hasn’t done that though. So there’s only two options really- he’s badly fucked up, or Carrot simply isn’t important enough to get the time devoted to her.